Invalidate cache when `--config-settings` change (#7139)
## Summary
If `--config-settings` are provided, we cache the built wheels under one
more subdirectory.
We _don't_ invalidate the actual source (i.e., trigger a re-download) or
metadata, though -- those can be reused even when `--config-settings`
change.
Closes https://github.com/astral-sh/uv/issues/7028.
