Where to Set Composer Environment Variables

I don't want to disable Xdebug, I don't care that it's slower, so I see there is an environment variable COMPOSER_DISABLE_XDEBUG_WARN and the docs mention setting it in the config.

So, I opened my global /Users/username/.composer/config.json and add:

{
    "config": {
        "COMPOSER_DISABLE_XDEBUG_WARN": true
    }
}

No change.

Where or how do I set this environment variable to get rid of this damn message?