Update cachix/install-nix-action action to v21 (#13531)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

.github/workflows/macos.yml

@@ -17,7 +17,7 @@ jobs:
       - name: Download Crystal source
         uses: actions/checkout@v3
 
-      - uses: cachix/install-nix-action@v20
+      - uses: cachix/install-nix-action@v21
         with:
           install_url: https://releases.nixos.org/nix/nix-2.9.2/install
           extra_nix_config: |