Reload env layer in install scripts after venv creation so `PYTHON`
points to the virtualenv instead of the system interpreter.
* Reload env in `python.sh` before installing dependencies
* Reload env in `pre-commit.sh` before installing hooks
* Reset env cache flags to force re-evaluation
* Keep centralized environment resolution (no hardcoding)
Fixes PEP 668 `externally-managed-environment` errors and missing
`pre_commit` module during hook installation.