You are in this scenario: Python: wrapped python packages don't propagate their PYTHONPATH · Issue #23676 · NixOS/nixpkgs · GitHub
One approach is a patch to the sources. Or, a PYTHONPATH needs to be set. For the latter, I’m not sure, but I think maybe this is your solution: Broken python path in custom python package - #2 by tobiasBora
It takes the value of $PYTHONPATH that’s set for the build shell, and puts it into a postFixup wrapper.