Use Pixi for environment and task management

[Yoshanuikabundi]

At the moment, I'm using SOAP (Yoshanuikabundi/snakes_on_a_plane) to manage my environments and run tasks in them. Pixi is SOAP, but with a whole company and actual community behind it. This should streamline... everything... and also act as a demonstration for Pixi in OpenFF

[Yoshanuikabundi]

Getting branches from Git for packages already installed through Conda gives a cryptic error and fails:

Error:
  × failed to solve the pypi requirements of 'default' 'osx-arm64'
  ├─▶ failed to resolve pypi dependencies
  ╰─▶ Because there is no version of openff-toolkit==0.16.7 and you require openff-toolkit==0.16.7, we can
      conclude that your requirements are unsatisfiable.

See eg. https://github.com/openforcefield/openff-pablo/actions/runs/12964648253/job/36163562278#step:4:9

This has been reported (see linked issue), see that issue for details. Tl;dr wontfix, but git rebuilds for conda packages are planned.

[mattwthompson]

Huh, based on the comment by the Prefix engineer, maybe this isn't possible unless everything is grabbed from PyPI? Just using one package from PyPI isn't a deal-breaker but I suspect that'll just bubble up the dependency tree