buildkite: don't run packetimpact test on old ubuntu agents

This test fails on 18.04 ubuntu agents.

PiperOrigin-RevId: 758760253

Author: avagin

.buildkite/pipeline.yaml

@@ -469,6 +469,8 @@ steps:
     command: make packetimpact-tests
     agents:
       <<: *ubuntu_agents
+      # This test is flaky on old agents.
+      cgroup: "v2"
 
   # Runtime tests.
   - <<: *common