Support pixi

[dometto]

pixi seems to be picking up a lot of traction. It's a fast alternative to conda, which also uses uv (as a rust library -- the tools are independent) for installing python packages. Probably too early to retire use of conda, but we could install it on VRE Lab / in our workbenches, and add something in our manuals.

Todo:

• Create a role that installs pixi as a global (for all users) tool
• Add this role in the right playbooks