Set QSG_RHI_BACKEND to opengl

in order to prevent a crash ancompanying the console output:
QQuickWidget is only supported on OpenGL.
Use QQuickWindow::setGraphicsApi() to override the default

Change-Id: Iab7c0cc486b150723000cc11ba4c6d9cd4be8b03
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/app/main.cpp

@@ -541,7 +541,7 @@ int main(int argc, char **argv)
     if (!qEnvironmentVariableIsSet("QT_OPENGL"))
         QCoreApplication::setAttribute(Qt::AA_UseOpenGLES);
 #else
-    qputenv("QT_QUICK_BACKEND", "opengl");
+    qputenv("QSG_RHI_BACKEND", "opengl");
 #endif
 
     if (qEnvironmentVariableIsSet("QTCREATOR_DISABLE_NATIVE_MENUBAR")