Update main repo ref

Author: ThomsonTan

.github/workflows/user_events.yml

@@ -27,7 +27,7 @@ jobs:
         uses: actions/checkout@v3
         with:
           repository: "open-telemetry/opentelemetry-cpp"
-          ref: "v1.18.0"
+          ref: "64a74bfadbdc10f8f8e3d0435b19d08de19537d1"
           path: "opentelemetry-cpp"
           submodules: "recursive"
       - name: setup dependencies