[8.x](backport #6978) Fold triggered pipelines under elastic-agent

[mergify]

What does this PR do?

In order for the Buildkite PR bot to be able to publish meaningful comments for failures, the pipelines need to be collapsed / uploaded rather than triggered, otherwise we lose the context and the ability to have the comment include specific failures for the failed steps in triggered pipelines.

This commit switches away from triggering to pipeline upload commands to enable this functionality.

Why is it important?

Allows enabling ELASTIC_PR_COMMENTS_ENABLED: 'true' only on the parent job

Logs/Screenshots

Before (note the null which points to a triggered pipeline):

After, with this PR when there are failures:

and when tests pass after retries:

---

This is an automatic backport of pull request #6978 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 33d0137 has failed:

On branch mergify/bp/8.x/pr-6978
Your branch is up to date with 'origin/8.x'.

You are currently cherry-picking commit 33d01371e.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   .buildkite/integration.pipeline.yml
	modified:   .buildkite/pipeline.yml

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   .buildkite/bk.integration.pipeline.yml

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[elasticmachine]

💔 Build Failed

• Buildkite Build
• Commit: 5b72530

Failed CI Steps

• Triggering Integration tests

History

cc @dliappis

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarQube

[pazone]

LGTM