[Build] Experiment detection for intermediate build

Setting up #65 and #33 gave me a thought.

I'll have to check the git commands again, though it is hard to get a list of changed files from the CI as the git history isn't pulled by default.

If we could, we could try and detect the modified experiments as part of a PR and have a separate job which uses that info to build/validate those experiments only (alongside the full build).

This could be useful as our all-up build grows to give quick status back on the immediate change. We could also use it to push these interim changes of packages to the PR specific feed to bring back that functionality we have in our main repo currently...

Thoughts?