Avoid hangs before exiting CLI (#4793)
## Summary
The resolver sometimes starts HTTP requests that end up not being
necessary. When dropping the Tokio runtime before exiting we currently
wait for those to complete. This can cause noticeable hangs in the CLI,
particularly when the runtime is blocked on slow DNS resolution.
Resolves https://github.com/astral-sh/uv/issues/4599.
## Test Plan
This change resolves any reproducible hangs for me locally.
