nixos/display-managers: update set-session for new "SessionType" property

GDM 40.1 switched from storing X11 sessions in the "XSession" property
on AccountService to "Session" with a "x11" "SessionType".

For compatibility reasons, we should set both, since AccountService
doesn't seem to provide the compatibility for us.

nixos/modules/services/x11/display-managers/set-session.py

···
                     f"Setting session name: {session}, as we found the existing wayland-session: {session_file}"
                 )
                 user.set_session(session)
+                user.set_session_type("wayland")
             elif is_session_xsession(session_file):
                 logging.debug(
                     f"Setting session name: {session}, as we found the existing xsession: {session_file}"
                 )
                 user.set_x_session(session)
+                user.set_session(session)
+                user.set_session_type("x11")
             else:
                 logging.error(f"Couldn't figure out session type for {session_file}")
                 sys.exit(1)