chore: pin GitHub Actions workflows to full commit SHAs

[irfanuddinahmad]

Pins all uses: action refs to their full commit SHA with the version tag preserved as a comment. Part of org-wide SHA-pinning migration: openedx/.github#165

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.90%. Comparing base (3b40107) to head (e08a94b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #458   +/-   ##
=======================================
  Coverage   95.90%   95.90%           
=======================================
  Files          18       18           
  Lines         488      488           
  Branches       55       55           
=======================================
  Hits          468      468           
  Misses         19       19           
  Partials        1        1           

Flag	Coverage Δ
unittests	95.90% <ø> (ø)

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