Only drop build directories on program exit (#14304)
## Summary
This PR ensures that we avoid cleaning up build directories until the
end of a resolve-and-install cycle. It's not bulletproof (since we could
still run into issues with `uv lock` followed by `uv sync` whereby a
build directory gets cleaned up that's still referenced in the `build`
artifacts), but it at least gets PyTorch building without error with `uv
pip install .`, which is a case that's been reported several times.
Closes https://github.com/astral-sh/uv/issues/14269.
