Implement a phase for the `imageBuilderBuild` task that is provided by
the `koji-image-builder` plugin, which schedules tasks to run with
`image-builder`.
This change is part of an accepted change proposal [1] for Fedora to use
`koji-image-builder` to build (some of) its variants.
[1]: https://fedoraproject.org/wiki/Changes/KojiLocalImageBuilder
Signed-off-by: Simon de Vlieger <supakeen@redhat.com>
