generalize lib/make-bootable-image.nix

nixos-modules/lib/make-bootable-image.nix

@@ -5,7 +5,7 @@ pkgs.stdenv.mkDerivation {
   name = "boot.qcow2";
 
   nativeBuildInputs = with pkgs.buildPackages; [
-    config.system.build.qemu
+    buildPackages.qemu_test
     utillinux
     zstd
   ];