consider schedule calculation time in estimation;

--- a/src/Pure/Tools/build_schedule.scala	Fri Dec 08 18:56:19 2023 +0100
+++ b/src/Pure/Tools/build_schedule.scala	Fri Dec 08 19:36:27 2023 +0100
@@ -980,7 +980,7 @@
         else {
           val start = Time.now()
 
-          val new_schedule = scheduler.build_schedule(state)
+          val new_schedule = scheduler.build_schedule(state).update(state)
           val schedule =
             if (_schedule.graph.is_empty) new_schedule
             else List(_schedule.update(state), new_schedule).minBy(_.end)(Date.Ordering)