ci: Migrate the Coverity workflow to a Github Action

[amyspark]

As per https://github.com/marketplace/actions/unofficial-coverity-scan

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2fe25db) 89.24% compared to head (2643386) 78.97%.

❗ Current head 2643386 differs from pull request most recent head 9f0886d. Consider uploading reports for the commit 9f0886d to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #86       +/-   ##
===========================================
- Coverage   89.24%   78.97%   -10.28%     
===========================================
  Files          47       47               
  Lines        3516     2797      -719     
  Branches      688      557      -131     
===========================================
- Hits         3138     2209      -929     
+ Misses        339      249       -90     
- Partials       39      339      +300     

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[dragonmux]

We'll need to do a test run of this to make sure it works (we'll push the coverity branch with it as a test to see) but this provisionally looks good - please rebase it on main for merge.