CI: fix decoupler config to use correct python

[mloubout]

fix decoupler to use sys.python (like for parallel) to avoid picking up wrong pytest

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 86.70%. Comparing base (32c2a2d) to head (c15759b).

Files	Patch %	Lines
conftest.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2382      +/-   ##
==========================================
- Coverage   86.82%   86.70%   -0.13%     
==========================================
  Files         234      234              
  Lines       43985    43986       +1     
  Branches     8126     8126              
==========================================
- Hits        38192    38136      -56     
- Misses       5086     5148      +62     
+ Partials      707      702       -5     

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