[Fixes #12855] Remove parallel upload default value from env_var

[mattiagiupponi]

ref #12855

Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.97%. Comparing base (5eaad8a) to head (1e0c5ad).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #12857   +/-   ##
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  Coverage   67.97%   67.97%           
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  Files         977      977           
  Lines       59040    59040           
  Branches     6897     6897           
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  Hits        40132    40132           
  Misses      17266    17266           
  Partials     1642     1642