PoC for maintenance tasks. Privileged builder with e.g. RW access for
/mnt/koji could do GC, run simplified kojira tasks, etc. Cron scripts
just trigger new task and it will be processed by standard koji
mechanisms than running separate tool on some other machine. In future
delayed tasks can be used, so cron/external machinery could be
completely dropped out.

We need special builder setting to be able to take up such jobs -
allow_maintenance_tasks here. Maybe we will need additional
permissions for maintenance builders. These could be set up with regular
permissions mechanism for builder user.

Sample maintenance tasks included - rewrite of koji-prune-scratch from
koji-tools.