Seems we are blowing the available disk space on some of the PPA builders,
leading to build failures and sometimes even builds looping. Collected
wisdom states that as long as we remain below 15GB we should be fine on
all builders, above that we are likely to fail. This patch cleans up some
of the build detrius as we proceed significantly reducing the footprint.
This is only done on buildds to allow the common use case of removing
the build stamps for incremental updates.
For the detail people the graph below shows before and after consumption
for both i386 and amd64 test builds: