refactor(docs): use var for output filepath

scripts/merge-i18n.escript

@@ -10,8 +10,9 @@ main(_) ->
     Conf = [merge(Conf0, Cfgs1),
             io_lib:nl()
            ],
-    ok = filelib:ensure_dir("apps/emqx_dashboard/priv/i18n.conf"),
-    ok = file:write_file("apps/emqx_dashboard/priv/i18n.conf", Conf).
+    OutputFile = "apps/emqx_dashboard/priv/i18n.conf",
+    ok = filelib:ensure_dir(OutputFile),
+    ok = file:write_file(OutputFile, Conf).
 
 merge(BaseConf, Cfgs) ->
     lists:foldl(