[ksmserver] Use UpdateLaunchEnvJob to sync SESSION_MANAGER env
ksmserver starts in plasma-session. It currently sets the SESSION_MANAGER env back to just kinit and plasma-session which is then used for anything else that plasma-session starts.
This is after start-plasma has synced the environment to DBus/systemd.
By using the shared UpdateLaunchEnvJob we can sync all these places.