Add env var to disable cache

[sir-sigurd]

Description

TODO

• Unit tests
• add unit test for PackageEntry.get_cached_path()
• Documentation
• Changelog entry

[codecov]

Codecov Report

Merging #2091 (7a7cee6) into master (9664258) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2091      +/-   ##
==========================================
+ Coverage   45.83%   45.94%   +0.10%     
==========================================
  Files         420      420              
  Lines       19915    19954      +39     
  Branches     2306     2306              
==========================================
+ Hits         9129     9168      +39     
  Misses       9796     9796              
  Partials      990      990              

Flag	Coverage Δ
api-python	89.54% <100.00%> (+0.07%)	⬆️
catalog	15.72% <ø> (ø)
lambda	92.72% <ø> (ø)

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Impacted Files	Coverage Δ
api/python/quilt3/packages.py	92.60% <100.00%> (+0.11%)	⬆️
api/python/quilt3/util.py	83.44% <100.00%> (+0.16%)	⬆️
api/python/tests/integration/test_packages.py	100.00% <100.00%> (ø)

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[dimaryaz]

This seems a little bit over-engineered... Ideally, we'd just download the manifest and immediately deserialize it, without even writing to a temporary file - or create a temporary file, but use its file descriptor instead of opening it again. But I guess that's not supported right now.

Fine for now, but would be nice to clean up some day.