Since shellManager was removed, we have a quite big regressions
of any other shell that isn't the desktop one not loading in any
way from SDDM, causing a big issue for plasma mobile or any shell
based on plasma mini shell, which we have several now not working
images based on.
This allows setting the plasma shell with an environment variable that
can be passed to startplasma (both wayland and x11) so we can have
dedicated session files for different shells