fix(pytest-matrix): seed pip into the uv venv

[ywatanabe1989]

uv venv installs no pip/setuptools/wheel, so any test that shells out to sys.executable -m pip dies with "No module named pip".

The per-repo workflows this reusable replaced all built their env with actions/setup-python + pip install, so pip was always present. Losing it is an unintended regression of the CI unification, not a test bug.

Evidence

Caught by scitex-ai/scitex-hub#433. Its pip-install e2e gate proves the production pip_install_user_app --no-deps --target argv shape. It passes on develop (retired setup-python workflow) and fails on all three legs under this reusable, on two different runners:

AssertionError: pip reported rc=0 but the package dir did not land at .../site-packages/scitex_live_paper_hub_app.
  --pip stderr--
  /tmp/scitex-ci-runner-work/spartan-pooled-cpu-01/scitex-hub/scitex-hub/.venv/bin/python: No module named pip

The job log's Installed 330 packages list contains no pip, hatchling, setuptools or wheel.

Fix

uv venv --seed. It only adds pip/setuptools/wheel — the set every non-uv venv already ships — so it cannot break existing callers.