Support environment variables to override USER & SYSTEM resource paths

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

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