Ensure Pip cache operations are atomic. (#1778)
Ensure Pip cache operations are atomic. (#1778)

By forcing the temporary directories Pip creates to be on the same
filesystem as the Pip cache, we indirectly ensure the caching Pip does
is atomic and safe in the presence of parallel Pex runs.

Fixes #1776
jsirois/pex,jsirois/pex,pantsbuild/pex,pantsbuild/pex