ENH: Use ExternalData_OBJECT_STORES from environment as default

As a side note, a merge request updating CMake ExternalData module
has also been pushed. But has pointed out by the CMake core team, doing
so would prevent CMake from detecting an environment changes.
See https://gitlab.kitware.com/cmake/cmake/merge_requests/255#note_199077

Suggested-by: Bradley Lowekamp <blowekamp@mail.nih.gov>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25547 3bd1e089-480b-0410-8dfb-8563597acbee