refactor(build): Deterministic code generation

The rebar plugin grpcbox_plugin always re-generates modules with a
timestamp, which makes the stamping script to always result in a
new stamp after each build.

Issue has been sent
https://github.com/tsloughter/grpcbox_plugin/issues/12

apps/emqx_exhook/rebar.config

@@ -1,7 +1,7 @@
 %%-*- mode: erlang -*-
 {plugins,
  [rebar3_proper,
-  {grpcbox_plugin, {git, "https://github.com/HJianBo/grpcbox_plugin", {branch, "master"}}}
+  {grpcbox_plugin, {git, "https://github.com/zmstone/grpcbox_plugin", {branch, "master"}}}
 ]}.
 
 {deps,

apps/emqx_exproto/rebar.config

@@ -9,7 +9,7 @@
             {parse_transform}]}.
 
 {plugins,
- [{grpcbox_plugin, {git, "https://github.com/HJianBo/grpcbox_plugin", {branch, "master"}}}
+ [{grpcbox_plugin, {git, "https://github.com/zmstone/grpcbox_plugin", {branch, "master"}}}
  ]}.
 
 {deps,