Read the build-system.requires during pixi init in some way

[olivier-lacroix]

During pixi init from a pyproject.toml, it would be nice if pixi read the build-system.requires in some way, and maybe created a 'build' environment that would be used to build python packages

Originally posted by @pavelzw in #1084 (comment)

[tdejager]

Sure, but then we also need to support no build isolation, so that uv actually uses that environment, and not solves its own and hence would never use the conda resolved one: #1124