Fix code coverage report uploads

[JeanChristopheMorinPerso]

I recently noticed that the code coverage report uploads were no more running. The cause was that we forgot to un-bake the GH_COV_OS in the workflow after we release 0.14.0.

[codecov-commenter]

Codecov Report

Merging #1189 (4ebff50) into main (4d2f668) will decrease coverage by 0.15%.
The diff coverage is 67.39%.

@@            Coverage Diff             @@
##             main    #1189      +/-   ##
==========================================
- Coverage   86.21%   86.06%   -0.16%     
==========================================
  Files         191      191              
  Lines       19047    19215     +168     
  Branches     2104     2291     +187     
==========================================
+ Hits        16422    16537     +115     
- Misses       2079     2125      +46     
- Partials      546      553       +7     

Flag	Coverage Δ
unittests	86.06% <67.39%> (-0.16%)	⬇️

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Impacted Files	Coverage Δ
src/py-opentimelineio/opentimelineio/__init__.py	100.00% <ø> (ø)
...-opentimelineio/opentimelineio/console/__init__.py	100.00% <ø> (ø)
tests/test_console.py	92.41% <50.00%> (-1.25%)	⬇️
...neio_contrib/adapters/advanced_authoring_format.py	81.42% <59.28%> (-5.00%)	⬇️
...elineio_contrib/adapters/tests/test_aaf_adapter.py	98.36% <100.00%> (+0.09%)	⬆️

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[meshula]

ah yes, I remember the conversation where we said we would do this, thanks for finding this omission!