[email protected]: start after /var/lib/machines is mounted (systemd#6079)

This fixes a race condition during boot, where an nspawn container would start
before /var/lib/machines got mounted resulting in a failure.

Author: atx

units/[email protected]

@@ -11,6 +11,7 @@ Documentation=man:systemd-nspawn(1)
 PartOf=machines.target
 Before=machines.target
 After=network.target systemd-resolved.service
+RequiresMountsFor=/var/lib/machines
 
 [Service]
 ExecStart=@bindir@/systemd-nspawn --quiet --keep-unit --boot --link-journal=try-guest --network-veth -U --settings=override --machine=%i