[9.2] [CI] Optionally pass along BUILDKITE_PULL_REQUEST* variables to triggered pipelines (#239465) (#239855)

# Backport

This will backport the following commits from `main` to `9.2`:
- [[CI] Optionally pass along BUILDKITE_PULL_REQUEST* variables to
triggered pipelines
(#239465)](#239465)

<!--- Backport version: 9.6.6 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sorenlouv/backport)

<!--BACKPORT [{"author":{"name":"Brad
White","email":"[email protected]"},"sourceCommit":{"committedDate":"2025-10-21T07:53:34Z","message":"[CI]
Optionally pass along BUILDKITE_PULL_REQUEST* variables to triggered
pipelines (#239465)\n\n## Summary\n\nCloses #239463\n- See issue for
examples and context\n\n###
Testing\nTriggered\nhttps://buildkite.com/elastic/kibana-pull-request/builds/352003
from\nhttps://buildkite.com/elastic/kibana-renovate-helper/builds/20306\n\n---------\n\nCo-authored-by:
kibanamachine
<[email protected]>","sha":"59b29f55e7fa2f8a4deff1f8db4faac6524f12f6","branchLabelMapping":{"^v9.3.0$":"main","^v(\\d+).(\\d+).\\d+$":"$1.$2"}},"sourcePullRequest":{"labels":["Team:Operations","release_note:skip","backport:all-open","ci:build-storybooks","v9.3.0"],"title":"[CI]
Optionally pass along BUILDKITE_PULL_REQUEST* variables to triggered
pipelines","number":239465,"url":"https://github.com/elastic/kibana/pull/239465","mergeCommit":{"message":"[CI]
Optionally pass along BUILDKITE_PULL_REQUEST* variables to triggered
pipelines (#239465)\n\n## Summary\n\nCloses #239463\n- See issue for
examples and context\n\n###
Testing\nTriggered\nhttps://buildkite.com/elastic/kibana-pull-request/builds/352003
from\nhttps://buildkite.com/elastic/kibana-renovate-helper/builds/20306\n\n---------\n\nCo-authored-by:
kibanamachine
<[email protected]>","sha":"59b29f55e7fa2f8a4deff1f8db4faac6524f12f6"}},"sourceBranch":"main","suggestedTargetBranches":[],"targetPullRequestStates":[{"branch":"main","label":"v9.3.0","branchLabelMappingKey":"^v9.3.0$","isSourceBranch":true,"state":"MERGED","url":"https://github.com/elastic/kibana/pull/239465","number":239465,"mergeCommit":{"message":"[CI]
Optionally pass along BUILDKITE_PULL_REQUEST* variables to triggered
pipelines (#239465)\n\n## Summary\n\nCloses #239463\n- See issue for
examples and context\n\n###
Testing\nTriggered\nhttps://buildkite.com/elastic/kibana-pull-request/builds/352003
from\nhttps://buildkite.com/elastic/kibana-renovate-helper/builds/20306\n\n---------\n\nCo-authored-by:
kibanamachine
<[email protected]>","sha":"59b29f55e7fa2f8a4deff1f8db4faac6524f12f6"}}]}]
BACKPORT-->

Co-authored-by: Brad White <[email protected]>

Author: kibanamachine

.buildkite/scripts/steps/renovate/trigger_pr.sh

@@ -11,4 +11,4 @@ for var in $(env | grep ^GITHUB_PR_ | cut -d= -f1); do
   GITHUB_ENV_VARS+=("$var=${!var}")
 done
 
-ts-node .buildkite/scripts/steps/trigger_pipeline.ts kibana-pull-request "$BUILDKITE_BRANCH" "$BUILDKITE_COMMIT" "" "${GITHUB_ENV_VARS[*]}"
+ts-node .buildkite/scripts/steps/trigger_pipeline.ts kibana-pull-request "$BUILDKITE_BRANCH" "$BUILDKITE_COMMIT" "" "${GITHUB_ENV_VARS[*]}" "true"

.buildkite/scripts/steps/trigger_pipeline.ts

@@ -7,6 +7,7 @@
  * License v3.0 only", or the "Server Side Public License, v 1".
  */
 
+import type { BuildkiteTriggerBuildParams } from '#pipeline-utils';
 import { BuildkiteClient } from '#pipeline-utils';
 
 const pipelineSlug = process.argv[2];
@@ -15,10 +16,19 @@ const commit = process.argv[4] || 'HEAD';
 const kibanaBuildId = process.argv[5] || '';
 // key=value pairs in space separated string
 const extraEnvVariables = process.argv[6] || '';
+const includeBuildkitePrVars = process.argv[7] === 'true' || process.argv[7] === '1';
 
 (async () => {
   try {
     const client = new BuildkiteClient();
+
+    const buildkitePrVars: Partial<BuildkiteTriggerBuildParams> = {};
+    if (includeBuildkitePrVars) {
+      buildkitePrVars.pull_request_id = process.env.BUILDKITE_PULL_REQUEST;
+      buildkitePrVars.pull_request_base_branch = process.env.BUILDKITE_PULL_REQUEST_BASE_BRANCH;
+      buildkitePrVars.pull_request_repository = process.env.BUILDKITE_PULL_REQUEST_REPO;
+    }
+
     const build = await client.triggerBuild(pipelineSlug, {
       commit,
       branch,
@@ -31,6 +41,7 @@ const extraEnvVariables = process.argv[6] || '';
         }, {}),
       },
       ignore_pipeline_branch_filters: true, // Required because of a Buildkite bug
+      ...buildkitePrVars,
     });
     console.log(`Triggered build: ${build.web_url}`);
     process.exit(0);