[8.15](backport #40990) Add Buildkite trigger context annotation

[mergify]

Proposed commit message

Added Buildkite annotation, for scenarios when build is triggered by GH comment without code changes

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.

Related issues

• Relates: https://github.com/elastic/ingest-dev/issues/3041

Logs

Buildkite builds: https://buildkite.com/elastic/beats/builds?branch=oakrizan%3Aadd-triggering-reason

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This is an automatic backport of pull request #40990 done by [Mergify](https://mergify.com).

[oakrizan]

clean backport