terminal: Fix detection of ignored python venv (#20227)

Closes #19227

Since items listed in `.gitignore` file are not included in a worktree,
python virtual environment cannot be detected until venv directory is
unfolded from project panel and forcefully added into worktree. I didn't
come up with anything better than scanning fs directly. I'm not sure how
it will affect remote development. if at all.

Release Notes:

- Fixed detection of `detect_venv.directories` ignored by a worktree