infra: add opts for tox builds ran in parallel

[AbeCoull]

Issue #, if available:

Description of changes:

Testing done:
Output still shows for all parallel envs ran.
tox -p auto

Merge Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (84c3bb7) to head (c67a0f2).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #759   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          135       135           
  Lines         8903      8903           
  Branches      1998      1998           
=========================================
  Hits          8903      8903           

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[speller26]

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