chore(CI): use tag of emqtt-bench in github actions

.github/workflows/run_relup_tests.yaml

@@ -39,7 +39,7 @@ jobs:
     - uses: actions/checkout@v2
       with:
         repository: emqx/emqtt-bench
-        ref: master
+        ref: 0.3.4 
         path: emqtt-bench
     - uses: actions/checkout@v2
       with: