Add vault bot to allowed list (elastic#190977)

## Summary

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gif.

Adding github vault bot to allowed list to allow triggering buildkite
builds.

Related to PR elastic/package-storage-infra#787
And issue: elastic/ingest-dev#3495

.buildkite/pull_requests.json

@@ -8,6 +8,7 @@
       "enabled": true,
       "allow_org_users": true,
       "allowed_repo_permissions": ["admin", "write"],
+      "allowed_list": ["elastic-vault-github-plugin-prod[bot]"],
       "set_commit_status": true,
       "commit_status_context": "kibana-ci",
       "build_on_commit": true,