[submitter] minor updates to environment variables and user selection

[fabiencastan]

This pull request makes minor updates to how environment variables and user selection are handled in the simpleFarmSubmitter.py file to improve flexibility and robustness.

• Environment variable handling:

  • Added support for passing the PROD and PROD_ROOT environment variables to the submitter's environment if they are present.

• User selection logic:

  • Changed the order of precedence for selecting the user: now prioritizes the FARM_USER environment variable over USER and the system's default user.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.56%. Comparing base (391afc7) to head (33bc3ae).
⚠️ Report is 3 commits behind head on develop.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2899   +/-   ##
========================================
  Coverage    79.56%   79.56%           
========================================
  Files           51       51           
  Lines         6977     6977           
========================================
  Hits          5551     5551           
  Misses        1426     1426           

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