fix(python plugin): set UV_PYTHON to python in .venv

[yemaney]

Fixes #2411

Summary

The UV_PYTHON env variables sets which python executable uv will use.

The current plugin sets the UV_PYTHON value to .devbox/nix/profile/default/bin/python which means uv pip install commands will try to install and save libraries under the .devbox/nix/profile/default/lib/python3.x/site-packages directory.

This will cause an error as the directory doesn't have write permissions because it exists under a folder that is a symlinked to a folder under the /nix/store path.

This change makes sure UV_PYTHON points to the python executable under .venv.
This will make uv pip install commands install libraries to the site-packages directory under .venv just like normal pip install would.

How was it tested?

1. devbox create uvtest --template python-pip
2. cd uvtest
3. devbox add uv
4. uv pip install fastapi

[mdomke]

It would be awesome to get this merged! All my Python projects suffer from this problem.