Use pixi for CIs

[ocefpaf]

Testing pixi for CIs instead of micromamba.

This env is quite complex and, at the moment, everything but iris can be installed. For some reason adding it sefgaults:

pixi add "iris>=3"       
 WARN The feature 'extras' is defined but not used in any environment
⠂ updating lock-file   [00:00:15] [────────────────────────────────────────]    0/33                                                                                                           
  ⠒ py310:linux-64       [00:00:15] resolving conda
  ⠒ py310:osx-64         [00:00:15] resolving conda
  ⠒ py311:linux-64       [00:00:15] resolving conda
  ⠒ py310:win-64         [00:00:15] resolving conda
  ⠚ py311:osx-64         [00:00:15] resolving conda
  ⠚ py311:win-64         [00:00:15] resolving conda
  ⠒ py312:linux-64       [00:00:15] resolving conda
  ⠚ py312:win-64         [00:00:15] resolving conda
  ⠚ py312:osx-64         [00:00:14] resolving conda
[1]    97011 killed     pixi add "iris>=3"