I took the liberty to open a MR to discuss this.
Added 71a4e0c1 to restrict execution of melodic jobs to branches named melodic-devel, and kinetic jobs to any other. This is far from ideal, because branches based of melodic-devel will still run the kinetic job. Maybe we should use some prefix for melodic-based branches while this is not the default supported distro?
Ran a couple of test jobs (one for melodic-devel named branch and one for other) and it seems fine.