ci: set cache to serve from

Author: sandydoo

.github/workflows/test.yml

@@ -126,7 +126,7 @@ jobs:
         run: |
           curl --location https://github.com/sandydoo/nar-toolbox/releases/download/v0.1.0/nar-toolbox-${{ matrix.system }} -O
           chmod +x ./nar-toolbox-${{ matrix.system }}
-          ./nar-toolbox-${{ matrix.system }} serve &
+          ./nar-toolbox-${{ matrix.system }} serve https://cache.nixos.org &
 
       - uses: cachix/install-nix-action@v30
         with: