d/rules: fix dh_installsystemd invocation

d/rules: fix dh_installsystemd invocation

the services should definitely not be stopped in preinst!

relevant debdiff with this change:

 Preinst files: lines which differ (wdiff format)
 ------------------------------------------------
 [-# Automatically added by dh_installsystemd/13.11.4-]
 [-if [ -z "${DPKG_ROOT:-}" ] && [ "$1" = upgrade ] && [ -d /run/systemd/system ] ; then-]
 [-      deb-systemd-invoke stop 'proxmox-backup-banner.service' 'proxmox-backup-daily-update.service' 'proxmox-backup-daily-update.timer' 'proxmox-backup-proxy.service' 'proxmox-backup.service' >/dev/null || true-]
 [-fi-]
 [-# End automatically added section-]

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>