Allow discovery of venv in VIRTUAL_ENV env variable (#16853)
## Summary
Fixes #16744
Allows the cli to find a virtual environment from the VIRTUAL_ENV
environment variable if no `--python` is set
## Test Plan
Manual testing, of:
- Virtual environments explicitly activated using `source .venv/bin/activate`
- Virtual environments implicilty activated via `uv run`
- Broken virtual environments with no `pyvenv.cfg` file
