build: use base rebar.conf when emqx.git is as dependency

rebar.config.erl

@@ -2,10 +2,15 @@
 
 -export([do/2]).
 
-do(_Dir, CONFIG) ->
+do(Dir, CONFIG) ->
+    case iolist_to_binary(Dir) of
+        <<".">> ->
             {HasElixir, C1} = deps(CONFIG),
             Config = dialyzer(C1),
-    maybe_dump(Config ++ [{overrides, overrides()}] ++ coveralls() ++ config(HasElixir)).
+            maybe_dump(Config ++ [{overrides, overrides()}] ++ coveralls() ++ config(HasElixir));
+        _ ->
+            CONFIG
+    end.
 
 bcrypt() ->
     {bcrypt, {git, "https://github.com/emqx/erlang-bcrypt.git", {branch, "0.6.0"}}}.