mehcode/config is looking for maintainers

[ellie]

We currently depend on this, and have an open issue due to how it handles env vars (#2012)

Ref: mehcode/config-rs#549

In order to fix #2012, I'm happy to

1. Fork, use the fork to fix our issue
2. Make a PR to upstream

However depending on the maintenance status of upstream, we'll potentially need to start finding alternatives. Though it does look like several people are willing to step up, and it may become part of rust-cli.