build: pin rebar3_hex plugin version 7.0.2

rebar.config

@@ -84,4 +84,4 @@
   emqx_exproto_pb % generated code for protobuf
 ]}.
 
-{project_plugins, [erlfmt]}.
+{project_plugins, [erlfmt, {rebar3_hex, "7.0.2"}]}.