More precise locking with --prefix option (#4506)
## Summary
In #4085, support was implemented for the `--prefix` option. When using
this option, however, a lock is either acquired on the virtualenv or
globally, preventing multiple installs to different `--prefix`s from the
same interpreter.
In this change, acquire the lock on just the prefix in question.
## Test Plan
Ran a `uv pip install` with `--prefix` and `RUST_LOG=trace` and observed
that the lock was acquired in the prefix.
