formula: add PIP_CACHE_DIR to build env

[branchvincent]

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This should give us a nice speed up when rebuilding the same pip package multiple times (esp useful to save CI time in python version bump PRs)

Timings for brew reinstall -s black:
Before (no cache): 47s
After (with cache): 28s

[MikeMcQuaid]

Great work and idea, thanks @branchvincent!