Change the ubuntu developer build to a namespace build.
We need to have namespace build coverage in CMake configurations
like in the qmake ones.
Task-number: QTBUG-84881
Change-Id: I429a11486bd9a0c60a3d0423a9a41dc436f32f1e
Reviewed-by: Simon Hausmann <hausmann@gmail.com>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>