Use pixi for simplified environment/CI management

[scottstanie]

• TODO:

  • need to change the documentation on developer setup
  • Need to find out if theres some easy way to sync an environment.yml files for people who still want to use conda/mamba, or if that's only possible to do manually

• See https://pixi.sh/dev/workspace/multi_environment/#initial-write-up for a much more complicated CI setup that replaces GHA matrices, which we could use to add back in the "lower bound" dep check CI time comparison:

https://github.com/isce-framework/dolphin/actions/runs/15509570404/job/43668729954?pr=602

https://github.com/isce-framework/dolphin/actions/runs/15784901726/job/44499060536?pr=608