ENH: Copy the generated Slicer package to the CircleCI artifact dir.

The package generated by a pull request can then be downloaded by members of
the Slicer GitHub organization that have signed up on CircleCI. See:

https://circleci.com/docs/build-artifacts/

From: Matt McCormick <matt.mccormick@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25033 3bd1e089-480b-0410-8dfb-8563597acbee