Partly addresses Core#137 (closed)
It install CMSSW and therein the JetToolbox, which successfully compiles.
The Core is not yet linked in CMSSW, as certain changes (among the ones in Core!170 (merged)) should be propagated to the Core before.
A couple of additional scripts are added to start an el8 session, to set up the CMS environment, and to compile CMSSW directly from EL9, so that the whole process happens in a nearly transparent way from the point of view of the user.