more robust re-use of $ISABELLE_TMP_PREFIX (amending c1597167563e);

--- a/src/Pure/System/isabelle_system.scala	Wed Mar 18 17:44:27 2020 +0100
+++ b/src/Pure/System/isabelle_system.scala	Wed Mar 18 18:06:36 2020 +0100
@@ -170,7 +170,7 @@
 
   /* tmp files */
 
-  private def isabelle_tmp_prefix(): JFile =
+  def isabelle_tmp_prefix(): JFile =
   {
     val path = Path.explode("$ISABELLE_TMP_PREFIX")
     path.file.mkdirs  // low-level mkdirs

--- a/src/Pure/Thy/sessions.scala	Wed Mar 18 17:44:27 2020 +0100
+++ b/src/Pure/Thy/sessions.scala	Wed Mar 18 18:06:36 2020 +0100
@@ -386,6 +386,8 @@
         if (required_theories.isEmpty) (ancestor.get, Nil)
         else {
           val other_name = info.name + "_requirements(" + ancestor.get + ")"
+          Isabelle_System.isabelle_tmp_prefix()
+
           (other_name,
             List(
               make_info(info.options,