With the change introduced by !3541 (merged) some of Gauss tests started to fail because the MagneticFieldSvc was initialized too late (wrt. GiGa... actually GaussGeo that is loaded by GiGa and loads some detector elements requiring the MagneticFieldSvc laready registered in the UpdateManagerSvc).
This simple change ensures that the MagneticFieldSvc is added very early in the list so that any user of it will find it already correctly initialized.