chore(ci): pass ref to system-tests reusable workflow

Author: nccatoni

.github/workflows/system-tests.yml

@@ -52,6 +52,7 @@ jobs:
       id-token: write
     with:
       library: cpp_httpd
+      ref: 1e5d6b7096279ca43ce4826fda3cc805635b63c1
       binaries_artifact: system_tests_binaries
       desired_execution_time: 300  # 5 minutes
       scenarios_groups: tracer-release