chore(renovate): pin GitHub actions

[mkniewallner]

This is considered a good practice, and is even reported by zizmor.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.1%. Comparing base (7847354) to head (6ca9ef2).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1161   +/-   ##
=====================================
  Coverage   93.1%   93.1%           
=====================================
  Files         37      37           
  Lines       1000    1000           
  Branches      99      99           
=====================================
  Hits         931     931           
  Misses        55      55           
  Partials      14      14           

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