Support conda_build_config_vars in rattler-build

[isuruf]

For example

conda_build:
  error_overlinking: true

I assume that this is ignored for now.

cc @wolfv

[wolfv]

Is this given as additional CLI flags to conda-build or as env vars? Or is this part of the "variant config"?

[wolfv]

I am assuming this comes from the conda_forge.yml.

[isuruf]

See https://github.com/conda-forge/conda-forge-ci-setup-feedstock/blob/main/recipe/conda_forge_ci_setup/build_utils.py#L112-L128