factorio: give advice on how to retain source tarball (#350475)

      nix-prefetch-url file://\''$HOME/Downloads/factorio_\''${releaseType}_x64_\''${version}.tar.xz --name factorio_\''${releaseType}_x64-\''${version}.tar.xz

    Note the ultimate "_" is replaced with "-" in the --name arg!

    If you go this route you might want to tell Nix to explicitly hold on to the

    source tarball. Otherwise it could get GC'd from the Nix store and you'd

    have to redownload it next time the package wants to rebuild to use a newer

    dependency. E.g. if you're using NixOS:

      system.extraDependencies = [

        factorio.src

      ];

  '';

  desktopItem = makeDesktopItem {