Allow multiple specific builds

Currently the option "specific build" allows a single number.  This was working ok for me when I had a multijob build with unique job names and using $TRIGGERED_BUILD_NUMBER_<job name> environment variable exported by the Parameterized Trigger plugin.

Now that I modified the build to trigger the same job twice with different parameters, I noticed and switched to using the $TRIGGERED_BUILD_NUMBERS_<job name> (pluralised), which evaluates to a comma-delimited list of build numbers.  

However, this does not work because the Copy Artifact plugin's "specific build" option expects a single parameter.

Perhaps it should allow multiple, so this scenario is supported?

(Note, I've attempted to use the "Build triggered by current Multijob build" option, but this also fails for reasons I can't currently determine.)