Use exec.d to set PYTHONPYCACHEPREFIX

[ryanmoran]

Summary

We will need to use an exec.d binary to set the PYTHONPYCACHEPREFIX with the dynamic value for $HOME. This is because the values placed into the normal environment variable file API are not interpolated/expanded before the process is executed.

The original implementation of this feature (#340) assumed this interpolation/expansion incorrectly and the bug was being resolved with a surprising and unexpected behavior. Specifically, the __pycache__ contents we being written to /workspace/'$HOME'/.pycache.

This resolved the immediate bug in that these files were no longer being written into what should have been a read-only layer, but then caused another bug in that it would easily crash a watchexec process that was restarting a process when there were modifications to the /workspace directory.

Use Cases

Enables paketo-buildpacks/python#478 to be merged.

Checklist

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• I have added an integration test, if necessary.
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