CI: clear workdir after job

gitlab-runner isn't great at clearing workdirs after a job (it actually clears it only when a new job from the same repository starts), wasting a fair amount of space (guesstimate of 12GB per runner per fork).
To reduce our impact on runners disks when our own CI isn't running, jobs should try to clear stuff when they end (for arti, we run cargo clean in post-script)

(this is actually a self note because I keep forgetting I have to work on that)