The update commands called bare 'pip' as fallback when uv wasn't found.
On modern Debian/Ubuntu enforcing PEP 668, this resolves to system pip
which refuses to install in an externally-managed environment.
Use sys.executable -m pip to ensure the venv's pip is used. Fixed in
both cmd_update and _update_via_zip (the PR only caught one instance).
Salvaged from PR #2655 by devorun. Fixes#2648.