Bump cachix/install-nix-action from 20 to 21 (#1062)

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 20 to 21.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](cachix/install-nix-action@v20...v21)

---
updated-dependencies:
- dependency-name: cachix/install-nix-action
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

.github/workflows/hlint.yml

@@ -16,7 +16,7 @@ jobs:
         uses: actions/checkout@v3
 
       - name: Install Nix
-        uses: cachix/install-nix-action@v20
+        uses: cachix/install-nix-action@v21
         with:
           nix_path: nixpkgs=channel:nixos-unstable
 

.github/workflows/nix.yml

@@ -18,7 +18,7 @@ jobs:
           - macos-latest
     steps:
     - uses: actions/checkout@v3
-    - uses: cachix/install-nix-action@v20
+    - uses: cachix/install-nix-action@v21
       with:
         nix_path: nixpkgs=channel:nixos-unstable
     - run: nix-build