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] <support@github.com>

.github/workflows/nix.yml

@@ -12,6 +12,6 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v3.0.2
-    - uses: cachix/install-nix-action@v17
+    - uses: cachix/install-nix-action@v18
     - run: nix build
     - run: nix flake check