fix parsing package outputs with rattler-build

[minrk]

rattler_build_conda_compat.render doesn't load the default variants the same way as conda_build.render

this loads and passes the variants dict, instead of just the list of variant files. It is unclear to me if this is truly a bug here, rattler_build_conda_compat, or in staged_recipes.

I believe this fixes conda-forge/staged-recipes#27817 (at least, I was able to run the failing step by hand and it succeeded where it failed before).

[beckermr]

We need to use conda build to combine the variants. I have a code snippet for this in the ci setup package.

[minrk]

Thanks, that's exactly what I was looking for but couldn't find. Copied from here.