dh_installsysusers is not working properly in debhelper(12.1).
Just hardcode these contents which are generated by debhelper(13.11.4)
in the postinst script.
Log:
Upgrade debian/compat to 12 to
let dh_installsystemduser auto enable and
start linglong-upgrade.timer and linglong-upgrade.service.
Signed-off-by: black-desk <me@black-desk.cn>