This moves the diff of outpaths into the outpaths job, mainly as a
preparation to allow future improvements. For example, this will allow
running the purity release checks only on changed outpaths instead of
the whole eval.
This also removes the inefficiency introduced in the last commit about
uploading the intermediate paths twice. Now, only the diff is passed on.
Also, technically, the diff is now run in parallel across 4 jobs. This
should be *slightly* faster than before, where outpaths from all systems
were combined first and then diffed. It's probably only a few seconds,
though.
Currently we need to rely on ofborg requesting reviews from package
maintainers, which takes a while with ofborg's eval queue. Since
recently we're doing faster evaluations with GitHub Actions, which contain all
necessary information to determine reviewers of changed packages the
same way ofborg does. This PR takes advantage of that.
