Ensure Pip cache operations are atomic.

[jsirois]

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]

I hand-tested this with the same repro.sh mentioned in the slack thread on this issue:

#!/usr/bin/env bash

set -euo pipefail

rm -rf ~/.pex/built_wheels/ ~/.pex/installed_wheels/ ~/.pex/wheels/ psutil-*.pex

pids=()
for i in $(seq 1 5); do
  python -m pex --no-wheel psutil -o psutil-$i.pex "$@" &
  pids+=($!)
done
for pid in ${pids[@]}; do
  wait ${pid}
done

That runs without failure (I waited for >10 attempts) via:

 count=0; while ./repro.sh; do echo "Attempt $((count+=1))"; done
Attempt 1
Attempt 2
Attempt 3
Attempt 4
Attempt 5
Attempt 6
Attempt 7
Attempt 8
Attempt 9
Attempt 10
Attempt 11
Attempt 12
Attempt 13
Attempt 14
Attempt 15
Attempt 16
Attempt 17
Attempt 18
Attempt 19
Attempt 20
Attempt 21
Attempt 22
Attempt 23
Attempt 24
Attempt 25
Attempt 26
Attempt 27
^C

[stuhood]

Do we think that this might have been related to pantsbuild/pants#15003?

[jsirois]

Yes, that Pants issue spawned this fix. I've updated / closed that ticket.