nixos/zfs: order pool sync services before final.target

If we don't do this, I think systemd can kill the service(s) before they
finish.

nixos/modules/tasks/filesystems/zfs.nix

@@ -761,6 +761,7 @@ in
           lib.nameValuePair "zfs-sync-${pool}" {
             description = "Sync ZFS pool \"${pool}\"";
             wantedBy = [ "shutdown.target" ];
+            before = [ "final.target" ];
             unitConfig = {
               DefaultDependencies = false;
             };