[8.19] (backport #11414) ci: use if_changed

[mergify]

What does this PR do?

remove dependency on the BK plugin monorepo-diff

Why is it important?

Faster builds
Remove the intermediate step to load the monorepo diff step

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

Related issues

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 #11414 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of ed59055 has failed:

On branch mergify/bp/8.19/pr-11414
Your branch is up to date with 'origin/8.19'.

You are currently cherry-picking commit ed5905518.
  (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)

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

no changes added to commit (use "git add" and/or "git commit -a")

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 succeeded, but was flaky

• Buildkite Build
• Commit: aa69ceb

Failed CI Steps

• x86_64:sudo: container - platform-ingest-elastic-agent-debian-13-1762801856

History

• 💔 Build #31166 failed b14e0de

cc @v1v