Allow using the stdlib caches in CI

[IanButterworth]

Avoid re-precompiling the stdlibs in CI

[IanButterworth]

This doesn't fix it all, but I think much of it might be unavoidable because it's using custom sysimages or intentionally isolating for tests... tbh I'm not sure

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.59%. Comparing base (686b959) to head (8fbb4c7).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1095   +/-   ##
=======================================
  Coverage   85.59%   85.59%           
=======================================
  Files           3        3           
  Lines         826      826           
=======================================
  Hits          707      707           
  Misses        119      119           

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