[8.19] (backport #8418) Fix pre-command to support extended testing

[mergify]

What does this PR do?

Fix a pre-command hook to allow BUILDKITE_PIPELINE_SLUG to match elastic-agent for PRs, and elastic-agent-extended-tests once it has merged to main

---

This is an automatic backport of pull request #8418 done by Mergify.

[mergify]

Cherry-pick of 97246c1 has failed:

On branch mergify/bp/8.19/pr-8418
Your branch is up to date with 'origin/8.19'.

You are currently cherry-picking commit 97246c150.
  (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/hooks/pre-command

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 Failed

• Buildkite Build
• Commit: 5fc3455

Failed CI Steps

• :pipeline:⬆️ Upload Pipeline: .buildkite/pipeline.yml
• :pipeline:⬆️ Upload Pipeline: .buildkite/pipeline.yml
• :pipeline:⬆️ Upload Pipeline: .buildkite/pipeline.yml

cc @michel-laterman

[michel-laterman]

We're going to include this backport as part of #8415 so we don't break the 8.19 branch