Possibly split build-repro job up into sub-jobs

In !2234 (merged) I enabled this for all MRs. In that MR it took 17 minutes. It would go faster (but use no less cpu time) if we parallelised it.

(AIUI there are some attmepts at caching target/ so it's possible that the runtime will improve as more of the workers' caches get populated.)

We shouldn't do anything to this until this month's release is done, though.