switch back to Python 3.12 for conda lock file creation due to mamba<2 pin

[valeriupredoi]

Description

mamba<2 is not compatible with Python 3.13, and we are pinning it until conda-lock==3 comes about, so I am switching the pin in the conda lock generator workflow, back to Python 3.12, so we can have updated conda lock files in the meantime. See https://github.com/ESMValGroup/ESMValCore/actions/runs/12102229199/job/33856299466

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.74%. Comparing base (0f9e888) to head (60ad1fd).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2606   +/-   ##
=======================================
  Coverage   94.74%   94.74%           
=======================================
  Files         252      252           
  Lines       14506    14506           
=======================================
  Hits        13744    13744           
  Misses        762      762           

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[valeriupredoi]

whoa two approves - I feel flattened, gents 😁