Add Jenkins job to update chroots
We should have a Jenkins job that updates the build chroots any now and then, see https://source.puri.sm/Librem5/deb-build-jobs/snippets/5
We should generate this out of the build-jobs-groovy since we know what we need there. A Jenkins job is preferable to a cron job here since we
- get visual feedback
- can make sure no other build is running