Speed up builds/optimize CI usage

What optimization(s) can we perform to speed up our builds and reduce our CI usage?

I know @jakirkham and @isuruf were talking about `--skip-existing` in https://github.com/conda-forge/conda-forge-pinning-feedstock/pull/94

I'm hoping this issue acts as a central listing of these kinds of optimizations and a discussion point for should we/how to implement the optimizations.

Although this is generally good, I think this would be especially helpful for our double migrations coming up.