build(deps): bump cachix/install-nix-action from 15 to 18

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 15 to 18.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](https://github.com/cachix/install-nix-action/compare/v15...v18)

---
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>

.github/workflows/nix-flake-check.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - uses: cachix/install-nix-action@v15
+      - uses: cachix/install-nix-action@v18
         with:
           nix_path: nixpkgs=channel:nixos-stable
           extra_nix_config: |