more correct indicator

--- a/src/Tools/Code/code_ml.ML	Fri Jun 14 12:29:50 2019 +0200
+++ b/src/Tools/Code/code_ml.ML	Sun Jun 16 16:40:57 2019 +0000
@@ -885,7 +885,7 @@
       evaluation_args = []})
   #> Code_Target.add_language
     (target_OCaml, {serializer = serializer_ocaml, literals = literals_ocaml,
-      check = {env_var = "ISABELLE_OCAMLEXEC",
+      check = {env_var = "ISABELLE_OCAMLFIND",
         make_destination = fn p => Path.append p (Path.explode "ROOT.ml")
           (*extension demanded by OCaml compiler*),
         make_command = fn _ =>