This allows parsing files like `/etc/nix/nix.conf` into the `NixConfig`
struct.
Change-Id: I90b25f43c429dd56127500ff5068e83852adee13
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11888
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: Brian Olsen <me@griff.name>