Write nix.settings to nix.custom.conf instead of nix.conf

[cole-h]

No description provided.

[cole-h]

(Waiting about 2 hours to see if the user in the nix-installer issue above tries it out and sees success, but otherwise I think this is the right approach and will merge with or without their feedback once that time limit has passed)

[teekennedy]

@cole-h I agree that this is the right approach, but unless I'm missing something, it only works for the initial darwin-rebuild switch.

Nix-darwin has a check to see whether /etc/nix/nix.custom.conf's hash matches one known to be generated by determinate nix installer 0.33+. On the first darwin-rebuild switch, the hash matches and nix-darwin overwrites the file. On subsequent runs, nix.custom.conf contains nix-darwin generated config and doesn't match the hash, so it errors out.

I've been working around this by running darwin-rebuild build --flake . && sudo mv /etc/nix/nix.custom.conf /etc/nix/$(date "+%Y-%m-%d_%H:%M:%S")-nix.custom.conf && darwin-rebuild switch --flake . to update my config.

Personally I'm in favor of merging this PR, but this won't be a full solution without some changes to nix-darwin.