[JENKINS-50585] Explicitly declared core builds against ATH smoke tests only

[olivergondza]

See JENKINS-50585. To be merged before jenkins-infra/pipeline-library#39.

Proposed changelog entries

• None, it is a refactoring with not impact intended

Submitter checklist

• JIRA issue is well described
• [ x Changelog entry appropriate for the audience affected by the change (users or developer, depending on the change). Examples
  * Use the Internal: prefix if the change has no user-visible impact (API, test frameworks, etc.)
• Appropriate autotests or explanation to why this change has no tests
• For dependency updates: links to external changelogs and, if possible, full diffs

Desired reviewers

@raul-arabaolaza, @jglick

[raul-arabaolaza]

I agree this change makes the metadata file more explicit even if I have concerns about pipeline-library#39