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

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>

Author: dependabot[bot]

.github/workflows/ci.yaml

@@ -39,7 +39,7 @@ jobs:
           echo ::set-output name=short::$(git rev-parse --short HEAD)
 
       - name: Install nix
-        uses: cachix/install-nix-action@v20
+        uses: cachix/install-nix-action@v21
         with:
           nix_path: nixpkgs=channel:nixos-unstable