COMP: Add python executable path to build tree launcher PATH settings.

This commit will allow to directly use python executable after
starting a terminal using "./Slicer --gnome-terminal" or after setting
the Slicer environment.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23633 3bd1e089-480b-0410-8dfb-8563597acbee