ci-package-builder: Link local child pipelines to parent

While propagation to child pipelines of static variables defined in the parent one is automatic, the propagation of the dynamic variables set in the dotenv artifacts does not happen unless the artifact is explicitly pulled in by the child pipeline with a snippet like:

needs:
  - pipeline: $PARENT_PIPELINE_ID
    job: prepare-build-env
    artifacts: true

For that to work, the parent pipeline need to actually populate $PARENT_PIPELINE_ID with its own id so let's do that.