ci(fix): ensure directory for i18n output exists

```
Run make eunit
escript: exception error: no match of right hand side value {error,enoent}
  in function  'merge-i18n_escript__escript__1669__814803__620426__2':main/1 (/__w/emqx/emqx/source/scripts/merge-i18n.escript, line 13)
  in call from escript:run/2 (escript.erl, line 750)
  in call from escript:start/1 (escript.erl, line 277)
  in call from init:start_em/1
  in call from init:do_boot/3
make: *** [Makefile:225: conf-segs] Error 127
```

scripts/merge-i18n.escript

@@ -10,6 +10,7 @@ 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).
 
 merge(BaseConf, Cfgs) ->