Move ki18n_install call to qt6 subdirectory
It is affected by KDEInstallDirs6 (e.g. KDE_INSTALL_LOCALEDIR), moving it to the qt6 subdirectory puts it after that.
It is affected by KDEInstallDirs6 (e.g. KDE_INSTALL_LOCALEDIR), moving it to the qt6 subdirectory puts it after that.