Expand environment variables in user-provided index URLs

Right now, we only support expansion (1) by the shell (of course) and (2) in requirements files. But the following don't expand:

- `cargo run pip install flask --index-url 'https://${PYPI_USERNAME}.com/repository/pypi-proxy/simple/flask/'`
- Anything in a `uv.toml` or `pyproject.toml` file
