Delay CI cache restore until the toolchain has been installed. (#28)

The [cache action's
docs](https://github.com/Swatinem/rust-cache/blob/68b3cb7503c78e67dae8373749990a220eb65352/README.md)
state:
> selecting a toolchain either by action or manual `rustup` calls should
happen before the plugin, as the cache uses the current rustc version as
its cache key

In practice we weren't affected too much, because it turns out the cache
key is also derived using all environment variables that have the prefix
`RUST`, which includes e.g. our `RUST_STABLE_VER`. So even if the key
was derived using the CI runner's pre-installed old Rust toolchain, our
env variable prevented most conflicts.

The cache docs also state that it does not cache, by cleaning the
following:
> Any files in ~/.cargo/bin that were present before the action ran (for
example rustc).

As we were installing the toolchain after the cache action, the
potential for caching the toolchain seems there. However the cache size
hasn't changed after this ordering change. Perhaps it does a simple path
check only and the pre-installed toolchain was in the same path.

In any case, better to invoke the cache action after the toolchain has
been installed, as the docs suggest.

.github/workflows/ci.yml

@@ -90,11 +90,6 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: restore cache
-        uses: Swatinem/rust-cache@v2
-        with:
-          save-if: ${{ github.event_name != 'merge_group' }}
-
       - name: install stable toolchain
         uses: dtolnay/rust-toolchain@master
         with:
@@ -107,6 +102,11 @@ jobs:
         with:
           tool: cargo-hack
 
+      - name: restore cache
+        uses: Swatinem/rust-cache@v2
+        with:
+          save-if: ${{ github.event_name != 'merge_group' }}
+
       - name: cargo clippy (no_std)
         run: cargo hack clippy --workspace --locked --optional-deps --each-feature --ignore-unknown-features --features libm --exclude-features ${{ env.FEATURES_DEPENDING_ON_STD }} --target x86_64-unknown-none -- -D warnings
 
@@ -122,11 +122,6 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: restore cache
-        uses: Swatinem/rust-cache@v2
-        with:
-          save-if: ${{ github.event_name != 'merge_group' }}
-
       - name: install stable toolchain
         uses: dtolnay/rust-toolchain@master
         with:
@@ -139,6 +134,11 @@ jobs:
         with:
           tool: cargo-hack
 
+      - name: restore cache
+        uses: Swatinem/rust-cache@v2
+        with:
+          save-if: ${{ github.event_name != 'merge_group' }}
+
       - name: cargo clippy
         run: cargo hack clippy --workspace --locked --target wasm32-unknown-unknown --optional-deps --each-feature --ignore-unknown-features --features std -- -D warnings
 
@@ -154,16 +154,16 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: restore cache
-        uses: Swatinem/rust-cache@v2
-        with:
-          save-if: ${{ github.event_name != 'merge_group' }}
-
       - name: install stable toolchain
         uses: dtolnay/rust-toolchain@master
         with:
           toolchain: ${{ env.RUST_STABLE_VER }}
 
+      - name: restore cache
+        uses: Swatinem/rust-cache@v2
+        with:
+          save-if: ${{ github.event_name != 'merge_group' }}
+
       - name: cargo test
         run: cargo test --workspace --locked --all-features
 
@@ -173,17 +173,17 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: restore cache
-        uses: Swatinem/rust-cache@v2
-        with:
-          save-if: ${{ github.event_name != 'merge_group' }}
-
       - name: install stable toolchain
         uses: dtolnay/rust-toolchain@master
         with:
           toolchain: ${{ env.RUST_STABLE_VER }}
           targets: wasm32-unknown-unknown
 
+      - name: restore cache
+        uses: Swatinem/rust-cache@v2
+        with:
+          save-if: ${{ github.event_name != 'merge_group' }}
+
       # TODO: Find a way to make tests work. Until then the tests are merely compiled.
       - name: cargo test compile
         run: cargo test --workspace --locked --target wasm32-unknown-unknown --all-features --no-run
@@ -197,11 +197,6 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: restore cache
-        uses: Swatinem/rust-cache@v2
-        with:
-          save-if: ${{ github.event_name != 'merge_group' }}
-
       - name: install msrv toolchain
         uses: dtolnay/rust-toolchain@master
         with:
@@ -213,6 +208,11 @@ jobs:
         with:
           tool: cargo-hack
 
+      - name: restore cache
+        uses: Swatinem/rust-cache@v2
+        with:
+          save-if: ${{ github.event_name != 'merge_group' }}
+
       - name: cargo check (no_std)
         run: cargo hack check ${{ env.RUST_MIN_VER_PKGS }} --locked --optional-deps --each-feature --ignore-unknown-features --features libm --exclude-features ${{ env.FEATURES_DEPENDING_ON_STD }} --target x86_64-unknown-none
 
@@ -225,11 +225,6 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: restore cache
-        uses: Swatinem/rust-cache@v2
-        with:
-          save-if: ${{ github.event_name != 'merge_group' }}
-
       - name: install msrv toolchain
         uses: dtolnay/rust-toolchain@master
         with:
@@ -241,6 +236,11 @@ jobs:
         with:
           tool: cargo-hack
 
+      - name: restore cache
+        uses: Swatinem/rust-cache@v2
+        with:
+          save-if: ${{ github.event_name != 'merge_group' }}
+
       - name: cargo check
         run: cargo hack check ${{ env.RUST_MIN_VER_PKGS }} --locked --target wasm32-unknown-unknown --optional-deps --each-feature --ignore-unknown-features --features std
 
@@ -252,14 +252,14 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - name: install nightly toolchain
+        uses: dtolnay/rust-toolchain@nightly
+
       - name: restore cache
         uses: Swatinem/rust-cache@v2
         with:
           save-if: ${{ github.event_name != 'merge_group' }}
 
-      - name: install nightly toolchain
-        uses: dtolnay/rust-toolchain@nightly
-
       # We test documentation using nightly to match docs.rs. This prevents potential breakages
       - name: cargo doc
         run: cargo doc --workspace --locked --all-features --no-deps --document-private-items -Zunstable-options -Zrustdoc-scrape-examples