nixos/google-compute-image: don't use <nixpkgs>

Avoid loading code from the NIX_PATH as it prevents pinning

nixos/modules/virtualisation/google-compute-image.nix

@@ -18,7 +18,7 @@ in
       rm $out/disk.raw
       popd
     '';
-    configFile = <nixpkgs/nixos/modules/virtualisation/google-compute-config.nix>;
+    configFile = ./google-compute-config.nix;
     format = "raw";
     inherit diskSize;
     inherit config lib pkgs;