Allow downloading wheels for metadata with `--no-binary` (#5707)
## Summary
We allow the use of (e.g.) `.whl.metadata` files when `--no-binary` is
enabled, so it makes sense that we'd also also allow wheels to be
downloaded for metadata extraction. So now, we validate `--no-binary` at
install time, rather than metadata-fetch time.
Closes https://github.com/astral-sh/uv/issues/5699.
