internal: Yarn should use local cache in CI so it actually saves/rest…

[ntucker]

…ores something

Also: @types/react no longer supports TypeScript 4.0 - so we need to install an older version to test that

[changeset-bot]

⚠️ No Changeset found

Latest commit: 74d5c10

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.49%. Comparing base (8a8634c) to head (74d5c10).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3052   +/-   ##
=======================================
  Coverage   98.49%   98.49%           
=======================================
  Files         122      122           
  Lines        2193     2193           
  Branches      448      448           
=======================================
  Hits         2160     2160           
  Misses         21       21           
  Partials       12       12           

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