Reuse compiled baseline in perf_compare

[JukkaL]

Comment by @huguesb in #14277:

The repeat checkout/compilation cost is a real problem for repeated measurements when iterating through possible approaches though. At the very least you'd want to keep the reference binary around for as long as possible instead of recompiling it every time.

We could cache build results. If we detect a previous build of the same target commit, reuse it instead of recompiling.