Non-flake git input and submodules

I’m wondering how to get nix to fetch ordinary git repo with submodules like

inputs = {
    river-src = {
      url = "github:riverwm/river";
      flake = false;
      # submodules = true; ?!

Is it even currently possible? If not, is there any workaround?

As far as I remember you can use submodules=true in the query string of the URL.

Though in my opinion flake inputs should not be used for anything but nix expressions.