CI: Add separate debug build with GCC 13

[danielhollas]

Hoping to make things less confusing by having a separate job that has -check=all and other runtime check flags.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.79%. Comparing base (094ba89) to head (23e8700).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #187      +/-   ##
==========================================
+ Coverage   91.90%   92.79%   +0.89%     
==========================================
  Files          47       47              
  Lines        6743     6678      -65     
  Branches      757      753       -4     
==========================================
  Hits         6197     6197              
+ Misses        534      469      -65     
  Partials       12       12              

Flag	Coverage Δ
unittests	20.64% <ø> (+0.21%)	⬆️

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see 10 files with indirect coverage changes

[JanosJiri]

This looks less confusing, thanks!