Introduce a full supermodule build in the CI

I have some demo devices running Linux desktop, and I regularly build all of Qt (including examples) and deploy to those devices.
Doing a simple configure && ninja in the qt5.git supermodule is the easiest way to build all of Qt, and it should be well supported. However, it breaks all the time for me, and I keep filing separate bugs for each breakage.

A supermodule build should be part of our CI.

• It takes a long time on a plain Linux desktop, but it should be fine on our CI if we assign a lot of CPUs to the VM and leverage Sccache.
• Only a "Build" workitem would be a good start. No need to run all tests, since they run in the submodule configurations.
• I would like to see this build happening on a new "Provision" configuration, that involves only a well known upstream distro (Debian?) without any custom-compiled packages.

Additionally, this build target can have extra restrictions set, that may catch bugs in the buildsystem that I've encountered repeatedly:

• No DISPLAY set in the environment
• Source directory read-only