noarch recipes are built on all 3 platforms in Pull Requests to staged-recipes

Why are noarch recipes submitted to staged-recipes built on Travis and AppVeyor? I couldn't find this discussed anywhere, so I wasn't sure if this was deliberate or if it is a problem of finding the time to update the build scripts.

Not only do these builds unnecessarily use CI time, but they can also fail. Currently any noarch R recipe submitted to conda-forge fails on AppVeyor. This is because we remove a Windows-specific selector to pass the linter checks.

This is happening to all recent noarch R recipes, but here is the PR where I discovered this: https://github.com/conda-forge/staged-recipes/pull/7128