Blender Git Loki

Windows/CMake:Support BLENDER_USER_SCRIPTS in IDE

This diff adds support for respecting the `BLENDER_USER_SCRIPTS`
environment variable when setting up the IDE environment.

Previously the scripts from the users profile folder were always
used even when this variable was set, leading to the possibility
of the wrong scripts being visible in the IDE.

Differential Revision: https://developer.blender.org/D9189
Reviewed By: Ray Molenkamp