How would this feature be useful?

uv pip --system and it's env variable equivalent UV_SYSTEM_PYTHON are meant for CI systems.

If Nox is run with UV_SYSTEM_PYTHON=true, it will crash with e.g.:

nox > Running session tests-3.8(opt_deps=[])
nox > Creating virtual environment (uv) using python3.8 in .nox/tests-3-8-opt_deps
nox > Command uv venv -p python3.8 /home/runner/work/stamina/stamina/.nox/tests-3-8-opt_deps failed with exit code 2:
error: the argument '--python <PYTHON>' cannot be used with '--system'

Describe the solution you'd like

UV_SYSTEM_PYTHON is always wrong within Nox so it should be removed before running uv.

Describe alternatives you've considered

I mean you can do it by hand but that's not great.

Anything else?

This is imho a damper on the usefulness of nox + uv. The option is somewhat new, but I'm sure people will start running into it soon enough.