ci: add zizmor job to lint GH Actions

[mccutchen]

Also, pin actions to latest releases.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.87%. Comparing base (f26ca58) to head (fc9353d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #252   +/-   ##
=======================================
  Coverage   94.87%   94.87%           
=======================================
  Files           9        9           
  Lines        1991     1991           
=======================================
  Hits         1889     1889           
  Misses         59       59           
  Partials       43       43           

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