[8.x](backport #42391) CI: x-pack/packetbeat: Enable GCP authentication for system tests

[mergify]

Proposed commit message

Enable GCP authentication for packetbeat windows system tests. Relates to Marc's PR

Extracted gcp_auth.sh from pre-command hook for simplicity
Ran the pipeline directly for this branch. The npcap is being successfully downloaded. Some follow-up work is expected

CC @marc-gr @oakrizan

Checklist

• 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.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Disruptive User Impact

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Closes https://github.com/elastic/ingest-dev/issues/4806

Use cases

Screenshots

Logs

---

This is an automatic backport of pull request #42391 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 5bd76b0 has failed:

On branch mergify/bp/8.x/pr-42391
Your branch is up to date with 'origin/8.x'.

You are currently cherry-picking commit 5bd76b0d5.
  (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/gcp_auth.sh
	modified:   .buildkite/x-pack/pipeline.xpack.packetbeat.yml

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

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