COMP: Ensure python is forced to Release when built on single config system

Following previous commit (r22797), since CMAKE_BUILD_TYPE is now properly
passed to all projects, it is important to force the value associated to
python to Release.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22798 3bd1e089-480b-0410-8dfb-8563597acbee