Even though individual USER/SYSTEM paths could be set using environment variables,
it wasn't possible to override the USER or SYSTEM paths.
This meant the result of `bpy.utils.resource_path('USER')` &
`bpy.utils.resource_path('SYSTEM')` could still be used by scripts,
making the Blender session potentially the default USER directory
(even when `BLENDER_USER_CONFIG`, `BLENDER_USER_SCRIPTS` &
`BLENDER_USER_DATAFILES` all point elsewhere).
Resolve by adding environment variables:
- BLENDER_USER_RESOURCES
- BLENDER_SYSTEM_RESOURCES
These will be used for `bpy.utils.resource_path('USER')` &
`bpy.utils.resource_path('SYSTEM')`, as well as a basis for user &
system directories, unless those environment variables are
set (`BLENDER_USER_*` or `BLENDER_SYSTEM_*`).
Resolves issue raised by T101389.
Example usage & output:
{P3225}
Reviewed By: brecht
Ref D16111