Only running coverage on ubuntu

[vineetbansal]

Looks like all runners in testing-pinned workflow run tests with coverage, but then discard the coverage results anyway. This is not a fix for the CI issue seen in PR #397, but should fix it anyway ;)

testing.yml still runs with coverage and uploads to codecov, like before.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.19%. Comparing base (bcd9bd6) to head (56dce99).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #408   +/-   ##
=======================================
  Coverage   86.19%   86.19%           
=======================================
  Files          14       14           
  Lines        1840     1840           
  Branches      318      318           
=======================================
  Hits         1586     1586           
  Misses        209      209           
  Partials       45       45           

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[rkingsbury]

Thanks @vineetbansal !