So you could just fix them (e.g. using /usr/bin/env bash etc. instead) and create a PR upstream. Alternative would be to create a nix package for the repository and use patchShebangs.
I got two answers even though I only asked one question In this case it seems to be easier to just fix the shebangs and propose changes upstream, but I didn’t know about patchShebangs so that is a plus.