chore: Specify emqtt-bench version

.github/workflows/run_fvt_tests.yaml

@@ -202,7 +202,7 @@ jobs:
         - uses: actions/checkout@v2
           with:
             repository: emqx/emqtt-bench
-            ref: master
+            ref: '0.3.3'
             path: emqtt-bench
         - uses: actions/checkout@v2
           with: