Draft: qtbase: Set an archdatadir
This way when qt submodules get built, they put such binaries under /app/lib such as /app/lib/qml.
Otherwise this needs to be workarounded by the apps' packaging.
Example https://github.com/flathub/org.kde.arianna/blob/master/org.kde.arianna.json#L17
It might be a bit too risky to include within 6.7, it could make sense to include right when we bump to 6.8.