How to set a default "source colour space"

I noticed the default value for "source colour space" is ACES2065, but I would like to default it to ACEScg. I couldn't find support for this within the OCIO env vars as I would for OCIO_ACTIVE_DISPLAYS, OCIO_ACTIVE_VIEWS etc.

Currently have to toggle this setting for each media imported

The issue names no files or tests. Start by locating where the source colour space is chosen during media import and where OCIO environment settings are handled; done means users can configure ACEScg as the default and imported media use it without toggling each item.

Written by the indexing model from the issue text.