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 94.06%. Comparing base (21d921c) to head (ee6ab84).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #445   +/-   ##
=======================================
  Coverage   94.06%   94.06%           
=======================================
  Files          31       31           
  Lines        3068     3068           
  Branches      191      191           
=======================================
  Hits         2886     2886           
  Misses        156      156           
  Partials       26       26           

Flag	Coverage Δ
unittests	94.06% <ø> (ø)

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