build(deps): bump cachix/install-nix-action from 20 to 21 (#7170)

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/cachix.yml

@@ -14,7 +14,7 @@ jobs:
       uses: actions/checkout@v3
 
     - name: Install nix
-      uses: cachix/install-nix-action@v20
+      uses: cachix/install-nix-action@v21
 
     - name: Authenticate with Cachix
       uses: cachix/cachix-action@v12