ci: Makefile: Expose CI_IMAGE_PREFIX and CI_IMAGE_TAG in 'ci-help'

ci: Makefile: Expose CI_IMAGE_PREFIX and CI_IMAGE_TAG in 'ci-help'

Using locally built images is a useful feature; our commentaries even
mention overriding them may be useful in some scenarios. Expose the
variables in the help to let users know they can use the feature.

Formatting would definitely break, so this patch adds more spacing for
proper alignment.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>