Pin Github Actions actions to their hashes

[jaimergp]

Part of napari/packaging#259

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.58%. Comparing base (b4859be) to head (3b48851).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #243   +/-   ##
=======================================
  Coverage   87.58%   87.58%           
=======================================
  Files          28       28           
  Lines        1144     1144           
=======================================
  Hits         1002     1002           
  Misses        142      142           

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[willingc]

Super happy to merge this @jaimergp. Here's to improved security.