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.

Changed files
+3
nixos
modules
services
x11
display-managers
+3
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)