more robust;

--- a/src/Pure/General/logger.scala	Mon Feb 13 10:39:49 2023 +0100
+++ b/src/Pure/General/logger.scala	Mon Feb 13 10:49:27 2023 +0100
@@ -35,7 +35,8 @@
 }
 
 class System_Logger extends Logger {
-  def apply(msg: => String): Unit =
+  def apply(msg: => String): Unit = synchronized {
     if (Platform.is_windows) System.out.println(msg)
     else System.console.writer.println(msg)
+  }
 }