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] <support@github.com>

.github/workflows/update.yml

@@ -10,7 +10,7 @@ jobs:
     name: Update flake.lock
     steps:
     - uses: actions/checkout@v4
-    - uses: cachix/install-nix-action@V27
+    - uses: cachix/install-nix-action@V28
       with:
         nix_path: nixpkgs=https://github.com/NixOS/nixpkgs/archive/nixpkgs-unstable.tar.gz
         extra_nix_config: |
@@ -83,7 +83,7 @@ jobs:
         echo
         swapon --show
         echo
-    - uses: cachix/install-nix-action@V27
+    - uses: cachix/install-nix-action@V28
       with:
         nix_path: nixpkgs=https://github.com/NixOS/nixpkgs/archive/nixpkgs-unstable.tar.gz
         extra_nix_config: |
@@ -117,7 +117,7 @@ jobs:
     - uses: actions/checkout@v4
       with:
         ref: "${{ github.ref_name }}"
-    - uses: cachix/install-nix-action@V27
+    - uses: cachix/install-nix-action@V28
       with:
         extra_nix_config: 'access-tokens = github.com=${{ github.token }}'
     - name: update database.nix