Pixi as backend #404
Description
After giving up on using rattler-build for source dependencies to be used by the parent Pixi workspace (#401), I started to wonder, why Pixi itself can't be used as a build backend?
So, I could imagine a parent Pixi workspace depending on Pixi packages in subdirectories, where the build process for those packages would be described by e.g. a Pixi task in this sub-manifest. The entire subdirectory could be hashed to avoid unnecessary rebuilds, and the build itself could happen in the parent .pixi/build/work/* subtree, like it currently happens with other backends.