proper relative path (see also df49ca5da9d0, 5b15eee1a661, etc.);

--- a/src/Pure/System/isabelle_system.ML	Mon Mar 01 14:58:00 2021 +0100
+++ b/src/Pure/System/isabelle_system.ML	Mon Mar 01 15:09:57 2021 +0100
@@ -82,7 +82,8 @@
 fun copy_file src dst = scala_function "copy_file" [src, dst];
 
 fun copy_file_base (base_dir, src) target_dir =
-  scala_function "copy_file_base" [base_dir, src, target_dir];
+  scala_function0 "copy_file_base"
+    [absolute_path base_dir, Path.implode src, absolute_path target_dir];
 
 
 (* tmp files *)