build(deps): bump cachix/install-nix-action from v12 to v13 (#19)

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from v12 to v13.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](https://github.com/cachix/install-nix-action/compare/v12...8d6d5e949675fbadb765c6b1a975047fa5f09b27)

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/nix.yml

@@ -21,7 +21,7 @@ jobs:
     runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v2
-      - uses: cachix/install-nix-action@v12
+      - uses: cachix/install-nix-action@v13
       - run: nix-shell --run ./ci.sh
         env:
           CI_REGISTRY_AUTH: '${{ secrets.REGISTRY_AUTH }}'