COMP: Fix packaging and upload for superbuild based extension

This commit fixes a regression introduced in r24289 (BUG: Update extension
build system to report packaging error on CDash.) ensuring the "packageupload"
target is built in the inner build directory.

Thanks to "Jinho Lee <jinho89@gmail.com>" for reporting the problem.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24303 3bd1e089-480b-0410-8dfb-8563597acbee