[CI] Trigger elastic agent builds from the root pipeline

Proposed commit message

Trigger elastic agent builds from the root pipeline instead of pull-requests.json

[ ] 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.