ci: our GTK tests were installing Cachix twice

.github/workflows/test.yml

@@ -376,15 +376,6 @@ jobs:
             -Dgtk-adwaita=${{ matrix.adwaita }} \
             -Dgtk-x11=${{ matrix.x11 }}
 
-      # Install Nix and use that to run our tests so our environment matches exactly.
-      - uses: cachix/install-nix-action@v30
-        with:
-          nix_path: nixpkgs=channel:nixos-unstable
-      - uses: cachix/cachix-action@v15
-        with:
-          name: ghostty
-          authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
-
   test-macos:
     runs-on: namespace-profile-ghostty-macos
     needs: test