testing/rbutil: force qt5
In some cases this tries to build with qt6 due to quazip-dev providing both qt5 and qt6 solinks, so let's make sure that this only ever tries qt5 for now.
In some cases this tries to build with qt6 due to quazip-dev providing both qt5 and qt6 solinks, so let's make sure that this only ever tries qt5 for now.