Bitmap import/export color space defaults when using OCIO env var not being set correctly

This is configurable via roles in the OCIO file itself. Substance Painter has those by default too, in e.g. `"C:\Program Files\Adobe\Adobe Substance 3D Painter\resources\ocio\aces_1.2\config.ocio"` the defaults are:

```
substance_3d_painter_bitmap_import_8bit: Utility - sRGB - Texture
substance_3d_painter_bitmap_import_16bit: Utility - sRGB - Texture
substance_3d_painter_bitmap_import_floating: Utility - Linear - sRGB
substance_3d_painter_bitmap_export_8bit: Utility - sRGB - Texture
substance_3d_painter_bitmap_export_16bit: Utility - sRGB - Texture
substance_3d_painter_bitmap_export_floating: ACES - ACEScg
substance_3d_painter_substance_material: Utility - sRGB - Texture
substance_3d_painter_standard_srgb: Utility - sRGB - Texture
```

For more details see the documentation here: https://helpx.adobe.com/substance-3d-painter/features/color-management/color-management-with-opencolorio.html which mentions those roles too.

Also explained to some extent here: https://mrlixm.github.io/blog/substance-painter-color-management

I do think the "locking" of those attributes seems like an odd UX - especially if those explicit roles are not defined I'd say those fields should've remained 'editable'. But I guess that might need an official bug report or feature request. I'd love to be able to specify a 'default aces 1.2 studio OCIO' using the 'OCIO' env var and allow a user to customize these settings for their project regardless.

I trying to setup an automated pipeline, that automatically sets projects up correctly with all the defaults so that artists don't have to think about all the settings.