Extends the uv-tool detection (briandevans, #29703) to cover the
remaining no-venv install layouts that hit the same uv 'No virtual
environment found' error:
- pipx-managed installs (sys.prefix under .../pipx/...) -> 'pipx upgrade',
matching scripts/auto-update.sh (pipx-detection idea from
inchargeautomation-lab, #29852)
- bare pip outside any venv -> 'uv pip install --system --upgrade'
- venv (launcher shim) keeps the VIRTUAL_ENV overlay from #35224 and never
gets --system, so the install always targets the venv, not system Python
The four branches are mutually exclusive; VIRTUAL_ENV is exported only for
the uv-pip-in-venv path (uv tool / pipx upgrade ignore it).
Co-authored-by: Joshua Kimbrell <incharge.automation@gmail.com>
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