ci: multiple fixes

[wpbonelli]

• skip vtk/pyvista pathlines example on windows for now, I think one of the windows-latest (2025) image updates today broke something which pyvista/vtk/opengl rely on
• switch mf6 CI tests to use the mf6 pixi environment, accommodating chore(dependencies): replace requirements files with conda env MODFLOW-ORG/modflow6-examples#275

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.1%. Comparing base (7eb7c88) to head (bf4df15).
Report is 12 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2503      +/-   ##
===========================================
- Coverage     76.1%    56.1%   -20.1%     
===========================================
  Files          296      640     +344     
  Lines        64215   124323   +60108     
===========================================
+ Hits         48885    69747   +20862     
- Misses       15330    54576   +39246     

see 475 files with indirect coverage changes

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