Add checked out ref for actions/checkout@v4

[chansuke]

When using third-party actions, it is recommended to, pin actions to a full length commit SHA for security reasons.
The latest release of actions/checkout@v4 introduces the ability to add ref: ${{ github.event.pull_request.head.sha }}, allowing it to reference the most recent commit in a pull request.

[HeavyWombat]

Thanks for the suggestion.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.97%. Comparing base (a8567c0) to head (87fd932).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #410   +/-   ##
=======================================
  Coverage   80.97%   80.97%           
=======================================
  Files          15       15           
  Lines        2092     2092           
=======================================
  Hits         1694     1694           
  Misses        326      326           
  Partials       72       72           

Flag	Coverage Δ
unittests	80.97% <ø> (ø)

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