Pin used GitHub actions

[Flamefire]

Use a specific commit instead of any (newer) release which might be compromised.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.16%. Comparing base (a557f20) to head (899e453).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #206      +/-   ##
===========================================
- Coverage    99.37%   99.16%   -0.21%     
===========================================
  Files           34       34              
  Lines         3340     3340              
===========================================
- Hits          3319     3312       -7     
- Misses          21       28       +7     

see 1 file with indirect coverage changes

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