Bind the io.systemd.SysUpdate.Notify.OnCompletedUpdate() method in the
sysext Varlink server. systemd-sysext provides a single Varlink service
covering both the sysext and confext image classes, so one notification
refreshes both (equivalent to "systemd-sysext refresh" plus
"systemd-confext refresh"). Hook a socket into
/run/systemd/sysupdate/notify/ via systemd-sysupdate-notify-sysext.socket,
enabled by default via the preset.
