only consider jobs late if they have ancestors (amending 12901c03b416);

--- a/src/Pure/Build/build_schedule.scala	Tue Jul 02 16:42:13 2024 +0200
+++ b/src/Pure/Build/build_schedule.scala	Tue Jul 02 17:38:28 2024 +0200
@@ -1156,9 +1156,8 @@
             (for {
               host <- _host_infos.hosts
               if !active_hosts0.contains(host.name)
-              jobs = _schedule.next(host.name, state)
-              ancestors = build_context.sessions_structure.build_requirements(jobs)
-              if ancestors.forall(ancestor =>
+              ancestors = _schedule.next(host.name, state).flatMap(_schedule.graph.imm_preds)
+              if ancestors.nonEmpty && ancestors.forall(ancestor =>
                 completed_since(ancestor) > build_options.seconds("build_schedule_inactive_delay"))
             } yield host).toSet