Bump cachix/install-nix-action from 20 to 21 (#578)

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](https://github.com/cachix/install-nix-action/compare/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/ci.yml

@@ -33,7 +33,7 @@ jobs:
           components: rustfmt
           profile: minimal
           toolchain: nightly
-      - uses: cachix/install-nix-action@v20
+      - uses: cachix/install-nix-action@v21
         if: ${{ matrix.os != 'windows-latest' }}
         with:
           nix_path: nixpkgs=channel:nixos-unstable