chore(quality-gates): update manual judgement on staging QGs (#196165)

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

- Similar to https://github.com/elastic/kibana/pull/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.*/