fix uploading C++ coverage for test_python workflow

[njzjz]

When using gcc-8 on Ubuntu, gcov-8 should be used instead of gcov. Thanks to https://stackoverflow.com/a/72435771/9567349

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +1.77 🎉

Comparison is base (5308c66) 76.70% compared to head (c81af1e) 78.48%.

Additional details and impacted files

@@            Coverage Diff             @@
##            devel    #2622      +/-   ##
==========================================
+ Coverage   76.70%   78.48%   +1.77%     
==========================================
  Files         233      233              
  Lines       24169    23869     -300     
  Branches     1693     1405     -288     
==========================================
+ Hits        18539    18733     +194     
- Misses       4512     4776     +264     
+ Partials     1118      360     -758     

see 64 files with indirect coverage changes

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