Bump cachix/install-nix-action from 20 to 21 (haskell#3612)

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

Author: dependabot[bot]

Diff for: .github/workflows/nix.yml

@@ -55,7 +55,7 @@ jobs:
     steps:
     - uses: actions/checkout@v3
 
-    - uses: cachix/install-nix-action@v20
+    - uses: cachix/install-nix-action@v21
       with:
         extra_nix_config: |
           experimental-features = nix-command flakes
@@ -87,7 +87,7 @@ jobs:
     steps:
     - uses: actions/checkout@v3
 
-    - uses: cachix/install-nix-action@v20
+    - uses: cachix/install-nix-action@v21
       with:
         extra_nix_config: |
           experimental-features = nix-command flakes