Try in xbin then.
/system/usr/we-need-root/su-backup
mount -o rw,remount -t ext4 /dev/block/system /system
cp /system/usr/we-need-root/su-backup /system/xbin/su
I ran into the issue that Rhubarb was having and figured something out. The permissions for su aren't set correctly when you copy the file. In the superuser app under info you can see the permissions are incorrectly set as "-rws--S---". The permissions should be "-rwsr-sr-x". After copying su to /system/xbin update the permissions using:
chmod 06755 /system/xbin/su
Superuser is then able to access the su binary giving back root. If you watch the Binary Updater process during an update you see it changes the permissions after copying to /system.