Provide a config setting for `--system-site-packages` for venv

[smackesey]

Summary

uv venv supports the --system-site-packages flag, but AFAICT there is no setting in the config file that can apply this by default to new venvs. It would be nice to have such a setting.

FWIW, my motivation here is that I use usercustomize.py to inject certain globally available logging/debugging functions into every python process during development (amazing how much better you can do than print()...). usercustomize.py is only discoverable if --system-site-packages is enabled. I am generating a lot of venvs, often non-interactively, in the course of development. So I need a way to have a uv user-level configuration setting to make sure these venvs are created with --system-site-packages.