Build both the Qt6 KF5 and KF6 host tools

Use the kf5 branch for the kf5-qt6 one and leave them in the same location to hopefully not break most of the existing setups.

Put kf6-qt6 into /opt/nativetooling6, which then requires updates to the KF6 templates and seed jobs to be found.

That hopefully decouples those two variants sufficiently enough.