cleanup target dir on job completion

related to tpo/tpa/team#40931 (closed)

on IRC I said a build used around 3GB of space, turns out I tried only on a transparent compression filesystem. The real size is actually a bit over 10GB.

The goal of this MR is to delete what's heavy before the end of the job, so it's not kept until a new job for the same project is scheduled. This may allow TPA to not clean cache as often and in general reduce the footprint of running CI.