download on separate thread;

--- a/src/Pure/System/isabelle_system.ML	Mon Mar 01 18:11:06 2021 +0100
+++ b/src/Pure/System/isabelle_system.ML	Mon Mar 01 18:24:27 2021 +0100
@@ -113,7 +113,7 @@
 
 fun download url =
   with_tmp_file "download" "" (fn path =>
-   (scala_function0 "download" [url, absolute_path path];
+   (Scala.function_thread "download" (cat_strings0 [url, absolute_path path]);
     File.read path));
 
 end;