delay ksplash until after env is set up

otherwise we can dbus invoke with the wrong environment. specifically
this happens with the latest qtbase changes that introduced color
picking support on wayland. when we start a qguiapplication with
incomplete environment that dbus invokes the xdg-portal system and that
in turn has an incomplete environment resulting in theming and the likes
not properly applying because the portal doesn't know that it runs
inside a plasma session.

qt/qt/qtbase@2dc083df

BUG: 458865
(cherry picked from commit 9bf0e56d)