ci: drop setup-micromamba

[wpbonelli]

732a55b introduced setup-uv and switched most jobs off setup-micromamba. Some were not changed because dependencies were only available from conda-forge, not PyPI

• windows pymetis
• arm mac vtk >=9.4.0

VTK is available from PyPI, and we can use the MF6 pixi rtd environment with setup-pixi where setup-micromamba is otherwise used, dropping setup-micromamba.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.4%. Comparing base (556c088) to head (2ac59a0).
⚠️ Report is 155 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2753      +/-   ##
===========================================
+ Coverage     55.5%    72.4%   +16.8%     
===========================================
  Files          644      675      +31     
  Lines       124135   132064    +7929     
===========================================
+ Hits         68947    95654   +26707     
+ Misses       55188    36410   -18778     

see 575 files with indirect coverage changes

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