To clarify, here is a snippet of the current SlicerLauncherSettings.ini found in the build tree :
[...]
[Environment]
additionalPathVariables=QT_PLUGIN_PATH,PYTHONPATH
[...]
[QT_PLUGIN_PATH]
1\path=<APPLAUNCHER_SETTINGS_DIR>/bin
2\path=/home/jcfr/Projects/Slicer-Qt5-VTK9-RelWithDebInfo/CTK-build/CTK-build/bin
3\path=/home/jcfr/Software/Qt5.11.1/5.11.1/gcc_64/plugins
size=3
[...]
This means that a env. variable QT_PLUGIN_PATH will be set before starting Slicer. Each one of these paths a subdirectory named designer. The same thing should be done in AdditionalLauncherSettings.ini.
Then the launcher will take of creating a unified environment before starting the application.