BUG: Ensure extension wizard can find git executable provided by Git Bash

This commit explicit set the environment variable "GIT_PYTHON_GIT_EXECUTABLE"
to ensure GitPython finds and uses the git executable provided by Git Bash.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23481 3bd1e089-480b-0410-8dfb-8563597acbee