To build the yaml files, we only need python3 and make.
All the other/old packages that got installed are needed for building
actual images, so get rid of them.
This should also significantly speed up the job.
While checking the 'artifacts' I noticed that git operations didn't
produce the output as expected, both in top comment as at the end where
build information is written to /etc/raspi-image-id.
That was caused by git not working as it wasn't installed.
cf2eb655d9