Use an external directory for container python cache

[tpoliaw]

With this set, the pyc files are created in a mirror directory instead
of the source tree. When running in a shared directory (as is the case
for the scratch area used by bluapi), this prevents files being created
with permissions that prevent other users removing them.

See python docs at
https://docs.python.org/3/using/cmdline.html#envvar-PYTHONPYCACHEPREFIX

Fixes #812

Instructions to reviewer on how to test:

1. Deploy and run on p46 with new scratch area
2. Check that there are no __pycache__ directories created in the scratch area

Checks for reviewer

• Would the PR title make sense to a user on a set of release notes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.46%. Comparing base (dc2015e) to head (b6f60a8).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #817   +/-   ##
=======================================
  Coverage   93.46%   93.46%           
=======================================
  Files          38       38           
  Lines        2111     2111           
=======================================
  Hits         1973     1973           
  Misses        138      138           

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[ZohebShaikh]

LGTM!

[callumforrester]

@tpoliaw did you find anything that worked for _version.py?

[tpoliaw]

@tpoliaw did you find anything that worked for _version.py?

Yes, installing blueapi manually in the virtualenv with pip sorted it. It's just that uv didn't seem to run whatever was needed when it created the venv.

Either way, I think it is unrelated to this PR.

[DiamondJoseph]

👍