more robust: avoid crash of sleep() for negative time;

--- a/src/Pure/System/bash.scala	Sat Jun 01 21:52:31 2024 +0200
+++ b/src/Pure/System/bash.scala	Sun Jun 02 13:36:24 2024 +0200
@@ -83,7 +83,7 @@
   }
   class Watchdog private(val time: Time, val check: Process => Boolean) {
     override def toString: String = "Bash.Watchdog(" + time + ")"
-    def defined: Boolean = !time.is_zero
+    def defined: Boolean = time > Time.zero
   }
 
   def process(script: String,