nix: place extra-prefixed settings after their non-prefixed variants

[sandydoo]

Fixes #626.

Essentially a copy of NixOS's workaround: NixOS/nixpkgs#278064

Description

Nix has somewhat surprising semantics when extra-prefixed settings are used in the same file as their non-prefixed siblings. Any extra-prefixed setting is ignored if it is placed before its respective non-prefixed setting.

Reproduce

Add an extra substituter to your config:

{
  nix.settings.extra-substituters = [
    "https://devenv.cachix.org"
  ];
}

The nix.conf in /etc/nix/nix.conf will be sorted alphabetically:

# WARNING: this file is generated from the nix.* options in
# your nix-darwin configuration. Do not edit it!
...
extra-substituters = https://devenv.cachix.org
...
substituters = https://cache.nixos.org

Verify that the extra substituter is missing with nix config show.

Nix ignores the --extra-prefixed keys if they're followed by their non-prefixed siblings, as documented in NixOS/nix#9487.