ci: pin GitHub actions to SHA

[phil-davis]

Pin the versions of GitHub actions using their SHAs.
The existing dependabot.yml should already check monthly for new releases of the actions and make a PR to bump the SHAs and the corresponding version number comment.

refs sabre-io/dav#1640

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.66%. Comparing base (013f8bb) to head (fb1aa15).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #768   +/-   ##
=========================================
  Coverage     98.66%   98.66%           
  Complexity     1757     1757           
=========================================
  Files            71       71           
  Lines          5163     5163           
=========================================
  Hits           5094     5094           
  Misses           69       69           

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