Write user config to nix.custom.conf, defer nix.conf to determinate when using determinate

[grahamc]

Description

Currently we have an untenable scenario where our nix configs are not "stable" with Determinate:

1. nix-installer writes out a nix.conf
2. determinate-nixd runs, moves the nix.conf to /etc/nix/nix.custom.conf and writes its own nix.conf

Then if the user runs the package again…

1. nix-installer merges its favorite configs with determinate-nixd's config, and updates nix.conf
2. determinate-nixd runs, moves nix.conf's contents into the end of nix.custom.conf, and writes its own nix.conf …

Every time the package is re-run, the nix.custom.conf grows bigger.

Let's fix some "original sin" where determinate-nixd is ultra-accommodating, and make everything a bit more Determinate.

Changes to nix-installer:

1. nix-installer will always write extra user-specified config to nix.custom.conf, and the generated nix.conf for upstream Nix will also use nix.custom.conf.
2. when installing Determinate, nix-installer won't write out its own nix.conf, and will defer that job to determinate-nixd init.

ref FH-539

Checklist

• Formatted with cargo fmt
• Built with nix build
• Ran flake checks with nix flake check
• Added or updated relevant tests (leave unchecked if not applicable)
• Added or updated relevant documentation (leave unchecked if not applicable)
• Linked to related issues (leave unchecked if not applicable)

Validating with install.determinate.systems

If a maintainer has added the upload to s3 label to this PR, it will become available for installation via install.determinate.systems:

curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix/pr/$PR_NUMBER | sh -s -- install

[grahamc]

Suggested change

	- uses: namespacelabs/breakpoint-action@v0
	if: failure()
	with:
	duration: 30m
	authorized-users: grahamc, cole-h

[grahamc]

Suggested change

	inputs.determinate-nixd-aarch64-linux = {
	url = "https://install.determinate.systems/determinate-nixd/pr/190/aarch64-linux";
	flake = false;
	};
	inputs.determinate-nixd-x86_64-linux = {
	url = "https://install.determinate.systems/determinate-nixd/pr/190/x86_64-linux";
	flake = false;
	};
	inputs.determinate-nixd-aarch64-darwin = {
	url = "https://install.determinate.systems/determinate-nixd/pr/190/macOS";
	flake = false;
	};