more robust, notably for "isabelle scala";

--- a/src/Pure/System/scala.scala	Fri May 22 11:30:06 2020 +0200
+++ b/src/Pure/System/scala.scala	Fri May 22 12:18:09 2020 +0200
@@ -25,7 +25,11 @@
         map(File.absolute_name)
 
     val class_path =
-      space_explode(JFile.pathSeparatorChar, System.getProperty("java.class.path", ""))
+      for {
+        prop <- List("scala.boot.class.path", "java.class.path")
+        path = System.getProperty(prop, "") if path != "\"\""
+        elem <- space_explode(JFile.pathSeparatorChar, path)
+      } yield elem
 
     (class_path ::: jar_dirs.flatMap(find_jars)).mkString(JFile.pathSeparator)
   }