build(deps): bump cachix/install-nix-action from V27 to 28

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from V27 to 28. This release includes the previously tagged commit.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](cachix/install-nix-action@V27...V28)

---
updated-dependencies:
- dependency-name: cachix/install-nix-action
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/flake-update.yml

@@ -17,7 +17,7 @@ jobs:
         with:
           token: "${{ secrets.GITHUB_TOKEN }}"
       - name: Install nix
-        uses: cachix/install-nix-action@V27
+        uses: cachix/install-nix-action@V28
         with:
           extra_nix_config: |
             auto-optimise-store = true
@@ -46,7 +46,7 @@ jobs:
         with:
           token: "${{ secrets.GITHUB_TOKEN }}"
       - name: Install nix
-        uses: cachix/install-nix-action@V27
+        uses: cachix/install-nix-action@V28
         with:
           extra_nix_config: |
             auto-optimise-store = true
@@ -91,7 +91,7 @@ jobs:
         with:
           token: "${{ secrets.GITHUB_TOKEN }}"
       - name: Install nix
-        uses: cachix/install-nix-action@V27
+        uses: cachix/install-nix-action@V28
         with:
           extra_nix_config: |
             auto-optimise-store = true
@@ -127,7 +127,7 @@ jobs:
         with:
           token: "${{ secrets.GITHUB_TOKEN }}"
       - name: Install nix
-        uses: cachix/install-nix-action@V27
+        uses: cachix/install-nix-action@V28
         with:
           extra_nix_config: |
             auto-optimise-store = true
@@ -159,7 +159,7 @@ jobs:
         with:
           token: "${{ secrets.GITHUB_TOKEN }}"
       - name: Install nix
-        uses: cachix/install-nix-action@V27
+        uses: cachix/install-nix-action@V28
         with:
           extra_nix_config: |
             auto-optimise-store = true