Fix CircleCI pixi setup

[bouweandela]

Description

Closes ESMValGroup/ESMValTool#4582

Also includes small improvements to storage of artifacts.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.48%. Comparing base (90cf5ad) to head (a7856b7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3219      +/-   ##
==========================================
- Coverage   96.50%   96.48%   -0.02%     
==========================================
  Files         282      282              
  Lines       16407    16407              
==========================================
- Hits        15833    15830       -3     
- Misses        574      577       +3     

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