Bump cachix/install-nix-action from 17 to 18

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 17 to 18.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](cachix/install-nix-action@v17...v18)

---
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]>

Author: dependabot[bot]

.github/workflows/nix.yml

@@ -19,7 +19,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - uses: cachix/install-nix-action@v17
+      - uses: cachix/install-nix-action@v18
         with:
           install_url: https://nixos-nix-install-tests.cachix.org/serve/i6laym9jw3wg9mw6ncyrk6gjx4l34vvx/install
           install_options: "--tarball-url-prefix https://nixos-nix-install-tests.cachix.org/serve"