Skip feedstock CI builds that don't produce binaries

I was thinking of other optimizations we could get. This seems to be a big one to me. Often there are PRs where we are adding a maintainer or re-rendering that won't actually ship a binary. As the CIs already ran on the PR, I see no point in doing it a second time for no package. The idea here would be to fast finish CIs by recognizing no binary will ship and thus not building at all.