fix(ci): force examples sequential run for luacov

[Aire-One]

For some times now, the coverage numbers are crazy. I found one reason with the way we run the examples tests in the CI: we have a -j24 to make them run faster. This is causing trouble with how the luacov.stats.out is written.

make distclean
mkdir build
cd build
cmake .. -DDO_COVERAGE=codecov
make check-examples -j24 
luacov
grep "tests/examples" luacov.report.out | wc -l # 38
make check-examples
luacov
grep "tests/examples" luacov.report.out | wc -l # 1164

This PR adds a new step for coverage where we run check-examples sequentially while preserving a faster step with the -j flag, so other jobs provides faster feedbacks.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.41%. Comparing base (efe027f) to head (bb5da1c).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4043       +/-   ##
===========================================
+ Coverage   54.51%   90.41%   +35.90%     
===========================================
  Files         264      938      +674     
  Lines       31249    60232    +28983     
  Branches     1139     1139               
===========================================
+ Hits        17034    54459    +37425     
+ Misses      13709     5267     -8442     
  Partials      506      506               

see 817 files with indirect coverage changes

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[Aire-One]

ahah! The coverage is back at 90%, which is what we had before 🎉

[actionless]

thanks for looking into it! 👌😸