Variants should work for all backend not only the recipe backend.

[tdejager]

Currently one can create variants in the pixi.toml file when using the build feature:

        [workspace.build-variants]
        python = ["3.10.*", "3.11.*"]

However, this only works with the recipe backend currently. This should be possible for all backends.

A pointer to get started on this is to look at the Output struct used by the backends and use it similar to the recipe backend. This could require a rattler-build change to get to work nicely.

[baszalmstra]

We can load/create a VariantConfig and call VariantConfig::combinations to get all the combinations of variants. We have to create outputs from that.