ci/nix: Drop weird magic URLs

I don't know what these are but I hope reverting to what
cachix/install-nix-action's documentation recommends will fix the errors
we're getting with presumably old Nix on new Ubuntu GitHub Actions
runners.

.github/workflows/nix.yml

@@ -14,11 +14,7 @@ jobs:
       - name: Install Nix
         uses: cachix/install-nix-action@v31
         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'
-          extra_nix_config: |
-            experimental-features = nix-command flakes
-            access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
+          github_access_token: ${{ secrets.GITHUB_TOKEN }}
       - name: Clone project
         uses: actions/checkout@v4
       - name: Build