Don't depend on LANG for running tests (#6853)

I have test failing because of incorrect output:
  'Aucun fichier ou dossier de ce nom'
instead of
  'No such file or directory'

Ensure dune is called with `LANG=C` to avoid such problem.

Author: psafont

dune

@@ -3,10 +3,13 @@
     (ocamlopt_flags (:standard -g -p -w -39))
     (flags (:standard -w -39))
   )
-  (dev (flags (:standard -g -w -39)))
+  (dev
+    (flags (:standard -g -w -39))
+    (env-vars (LANG C)))
   (release
     (flags (:standard -w -39-6@5))
-    (env-vars (ALCOTEST_COMPACT 1))
+    (env-vars (ALCOTEST_COMPACT 1)
+              (LANG C))
   )
 )