chore(quality-gates): update manual judgement on staging QGs

  - For now, only run the staging manual judgement on slice:
  'staging-ds-2'
  - Since slices can be grouped as a comma delimited list, we're choosing to
  be overly cautious with the regex

.buildkite/pipelines/quality-gates/emergency/pipeline.tests-staging.yaml

@@ -46,3 +46,4 @@ steps:
         command: "make -C /agent trigger-manual-verification-phase"
         agents:
           image: "docker.elastic.co/ci-agent-images/manual-verification-agent:0.0.6"
+        if: build.env("DEPLOYMENT_SLICES") =~ /.*staging-ds-2.*/