Improve GitHub Actions workflow security

[jwodder]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.31%. Comparing base (3e779cf) to head (3ee9a48).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #39   +/-   ##
=======================================
  Coverage   99.31%   99.31%           
=======================================
  Files           6        6           
  Lines         583      583           
  Branches      117      117           
=======================================
  Hits          579      579           
  Misses          3        3           
  Partials        1        1           

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