Find the Pipenv venv that .env and old Pipenv use

Agent mode and hosted stale-install checks now find the venv Pipenv
really uses in two cases where they used to miss it, patch the system
interpreter instead, and let VEX attest not_affected:

- WORKON_HOME, PIPENV_CUSTOM_VENV_NAME or PIPENV_VENV_IN_PROJECT set in
  the project's .env (or PIPENV_DOTENV_LOCATION), which every Pipenv
  command loads before it picks the venv (#546).
- An explicit "not in project" setting next to a ./.venv directory.
  Only Pipenv 2023.11.14+ skips ./.venv then; 2018.11 to 2023.10.24
  still use it, so both venvs are now patched (#645).

Assisted-by: Claude Code:claude-opus-5-5

Author: claude