Add a new Ubuntu 20.04 host to be used for the qemu configs.
We might be able to use the RHEL 7.6 host artifacts for qemu target
configs, but i'd rather we test on the same distro / OS for both
host and target.
Use the macOS host for iOS builds.
Task-number: QTBUG-85623
Change-Id: I6305f924b843a5a832bd94e5060211583c9c7f98
Reviewed-by: Cristian Adam <cristian.adam@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>