412374: repo2runnable task does not set

IArtifactRepository.PROP_RUNNABLE by default

the change applies only to the ant task. if the flagAsRunnable is set to
false, the flag is not set at all.

Bug: https://bugs.eclipse.org/bugs/show_bug.cgi?id=412374
Signed-off-by: Krzysztof Daniel <kdaniel@redhat.com>