RFC: Streamlining build pipeline by building only what changes

[jerdog]

We've slimmed down the build times to ~7min, but I believe there are opportunities to slim down even further using caching and sparse-checkout in the build process. Here's the theory:

• Caching: we identify specific folders that don't change between builds (like past years, and/or depending on outcome of RFC - Older event archiving #7) and unless that folder is updated we will just use the cache.
• Sparse-checkout: potential to use a variable based on which part of the repo has been edited as part of the PR and do a git checkout of just those files to then test against. This may not be as useful or worth the effort if we can use some caching.

This is a definite open discussion, and I will try to work on a POC of what at least the caching would look like.