Simple support for Qt 6 in CI

For Qt 6, we will need to work a bit differently in CI than we do with Qt 5.x, as we are expecting somewhat larger changes going into the branch that will break dependencies between modules.

Handling this in the current way, where we always compile modules against the latest passing qt5.git integration would slow us down too much.

The best solution (and easiest to implement) for the short term would be to add a mode (that we could ideally turn on/off with some tag in qt5.git for the relevant branch) where we compile and test against the latest heads of the branch instead of the sha1's from qt5.git.

This is basically the mode we initially had for COIN. It's disadvantage is that it requires more time and VMs for leaf modules. For modules that only depend on qtbase, there is no difference in load to the system and CI times. And since most of the initial Qt 6 development will be limited to qtbase and qtdeclarative this mode should work very well there.