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

  This is a build/promotion system change only, there is no code related
  changes here.

  - Similar to elastic#195944, but for
  the non-emergency pipeline in staging
  - 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/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.*/