cmake: update command to find python-site-packages
cmake: update command to find python-site-packages

https://stackoverflow.com/a/40006251
Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master