ci: test pixi build in CI

[ruben-arts]

Description

This adds a CI job to build pixi using pixi build and uploads it to https://prefix.dev/pixi-ci with pixi upload.

TODO:

• Use latest pixi (main) as we need the solve-group fix.
• Add default compiler variants to the rust backend to avoid issues with vs2017
• Add automated rust_compiler_version variant from rust-toolchain file to the rust backend.
• Add the ability to specify the profile that you want to build from CLI

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Tools: Claude

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.