Add cache for Micromamba installation

This endpoint sometimes fails to resolve, so caching should help. Cache
the entire ~/.local/bin folder to keep things simple.

.github/workflows/ci.yaml

@@ -38,10 +38,21 @@ jobs:
     name: build and test (os=${{ matrix.os }})
     runs-on: ${{ matrix.os }}
     steps:
+      - name: Cache ~/.local/bin
+        id: cache-local-bin
+        uses: actions/cache@v4
+        with:
+          path: ~/.local/bin
+          # Cache should be valid for a maximum of 7 days.
+          # <https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#usage-limits-and-eviction-policy>
+          key: ${{ runner.os }}-local-bin
+
       - name: Install Micromamba
-        run: |
-          curl -fsSL --proto 'https=' https://micro.mamba.pm/install.sh | bash
-          echo ~/.local/bin >> "$GITHUB_PATH"
+        if: steps.cache-local-bin.outputs.cache-hit != 'true'
+        run: curl -fsSL --proto 'https=' https://micro.mamba.pm/install.sh | bash
+
+      - name: Add ~/.local/bin to path
+        run: echo ~/.local/bin >> "$GITHUB_PATH"
 
       - uses: actions/checkout@v3