BUG: Fix PYTHONPATH in installation so that vtk can be imported

Sam Horvathrequested to merge
github/fork/cquammen/4556-add-to-PYTHONPATH into master

Created by: cquammen

Add an additional path (different on each OS) so that the vtk Python package can be found.

Fixes https://issues.slicer.org/view.php?id=4556.

Merge request reports