src/Pure/Tools/build_job.scala

--- a/src/Pure/Tools/build_job.scala	Mon Jun 28 20:52:31 2021 +0200
+++ b/src/Pure/Tools/build_job.scala	Wed Jun 30 11:35:07 2021 +0200
@@ -7,6 +7,8 @@
 package isabelle
 
 
+import java.util.HashMap
+
 import scala.collection.mutable
 
 
@@ -217,9 +219,8 @@
       val base = deps(parent)
       val result_base = deps(session_name)
 
-      val env =
-        Isabelle_System.settings() +
-          ("ISABELLE_ML_DEBUGGER" -> options.bool("ML_debugger").toString)
+      val env = new HashMap(Isabelle_System.settings())
+      env.put("ISABELLE_ML_DEBUGGER", options.bool("ML_debugger").toString)
 
       val is_pure = Sessions.is_pure(session_name)