Switch build_scenerios_on_region to return list for caching

[kddejong]

Issue #, if available:
fix #3495

Description of changes:

• Switch build_scenerios_on_region to return list for caching

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.39%. Comparing base (a896807) to head (acc71b8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3498      +/-   ##
==========================================
- Coverage   93.40%   93.39%   -0.02%     
==========================================
  Files         325      325              
  Lines       11091    11087       -4     
  Branches     2328     2327       -1     
==========================================
- Hits        10360    10355       -5     
  Misses        410      410              
- Partials      321      322       +1     

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