nixos/nixpkgs: Properly validate and merge nixpkgs config

[dasJ]

Description of changes

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[roberth]

I'm debating whether it should be this or types.deferredModule, ie pass a whole unevaluated (sub)module to Nixpkgs, and therefore also, make the Nixpkgs entrypoint accept a module for config.

The latter is somewhat more in the spirit of turning the "Nixpkgs invocation interface" entirely into a module, which is an admittedly unclear goal of #231940.

[infinisil]

allowUnfreePredicate should have a better type:

# configuration.nix
{ lib, ... }: {
  imports = [
    {
      nixpkgs.config.allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
        "zsh-abbr"
      ];
    }
    {
      nixpkgs.config.allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
        "slack"
      ];
    }
  ];
}

$ nix-instantiate --eval nixos --arg configuration ./configuration.nix -A config.nixpkgs.config.allowUnfreePredicate
<LAMBDA>

$ gh pr checkout https://github.com/NixOS/nixpkgs/pull/272686
Switched to branch 'feat/type-nixpkgs-config'

$ nix-instantiate --eval nixos --arg configuration ./configuration.nix -A config.nixpkgs.config.allowUnfreePredicate
error: The option `nixpkgs.config.allowUnfreePredicate' is defined multiple times while it's expected to be unique.

Definition values:
- In `/home/tweagysil/src/nixpkgs/main/configuration.nix': <function>
- In `/home/tweagysil/src/nixpkgs/main/configuration.nix': <function>
Use `lib.mkForce value` or `lib.mkDefault value` to change the priority on any of these definitions.

[roberth]

Haskell would call that Any, which adds "or" monoid semantics to Bool. Just a mergeable bool type.
Let's keep something more recognizable in the name though.
functionTo boolOr?
functionTo boolWithOr?
functionTo disjunction?
functionTo disjunctiveBool?
functionTo anyBool?

[infinisil]

#272764

Edit: Now merged, lib.types.boolByOr can (and should) now be used for the allowedInsecurePredicate and allowedUnfreePredicate options :)