Reduce Jenkins CI disk usage

See also eclipsefdn/helpdesk#5250 (closed)

Extends !245 (closed), which directly targeted the main branch. Instead merge to develop, as there's much more activity there. My assumption is that every MR targeting develop will take over the settings - at least MRs from committers.

I'll abandon !245 (closed), as it will lead to a merge conflict later on (even when only a trivial one).