feat(mix): set `snk_kind` from `$kind` to `msg` in Mix build

Log before:

```
2022-09-26T15:51:30.982183-03:00 [warning] '$kind': last_will_testament_publish_denied, clientid: myclient, line: 2132, mfa: emqx_channel:publish_will_msg/2, peername: 127.0.0.1:35228, topic: $SYS/lwt
```

Log after:

```
2022-09-26T16:25:50.799914-03:00 [warning] clientid: myclient, line: 2132, mfa: emqx_channel:publish_will_msg/2, msg: last_will_testament_publish_denied, peername: 127.0.0.1:47080, topic: $SYS/lwt
```

build

@@ -364,6 +364,7 @@ export_release_vars() {
   # compiling the project, so that `emqx_release.erl' picks up
   # `emqx_vsn' as if it was compiled by rebar3.
   erl_opts+=( "{compile_info,[{emqx_vsn,\"${PKG_VSN}\"}]}" )
+  erl_opts+=( "{d,snk_kind,msg}" )
 
   ERL_COMPILER_OPTIONS="[$(join , "${erl_opts[@]}")]"
   export ERL_COMPILER_OPTIONS