[9.1] (backport #11612) ci(packaging): let's test the packaging pipeline on PRs too

[mergify]

What does this PR do?

This pull request introduces an early trigger for the Elastic Agent Package pipeline in the Buildkite CI process, it runs
the https://buildkite.com/elastic/elastic-agent-package with dry-run using the versions defined at .package-version

Why is it important?

aiming to catch integration issues earlier during pull request builds.

Checklist

• I have read and understood the pull request guidelines of this project.
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool
• I have added an integration test or an E2E test

Disruptive User Impact

How to test this PR locally

Only in the CI, now it triggers a new step if changes are made in the main pipeline or any other related files with the packaging.

In this case, it ran https://buildkite.com/elastic/elastic-agent-package/builds/8694

Related issues

• Docker builds failing with invalid tar header or EOF errors #11604

Questions to ask yourself

• How are we going to support this in production?
• How are we going to measure its adoption?
• How are we going to debug this?
• What are the metrics I should take care of?
• ...

---

This is an automatic backport of pull request #11612 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of cffc2cd has failed:

On branch mergify/bp/9.1/pr-11612
Your branch is up to date with 'origin/9.1'.

You are currently cherry-picking commit cffc2cdff.
  (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:
	new file:   .buildkite/scripts/steps/trigger-elastic-agent-package.sh

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

[mergify]

This pull request has not been merged yet. Could you please review and merge it @v1v? 🙏

[elasticmachine]

💔 Build Failed

• Buildkite Build
• Commit: 347a55e

Failed CI Steps

• 📦 amd64: Containers
• 📦 amd64: FIPS Containers
• ESS FIPS stack cleanup

History

• 💔 Build #31622 failed cfe59e2
• 💔 Build #31547 failed d1c7c08
• 💔 Build #31545 failed 0e46f23

cc @v1v