nixos: Add system.build.{toplevel,installBootLoader}, improve error message

[roberth]

Motivation for this change

• Undocumented options are bad, even more so when they're important ones like toplevel
• Bad error message

This is the successor of #155883

Things done:

• add temporary helper function
• add types.unique { message } type
• make system.build.toplevel non-internal
• use types.unique to improve error message:

Original error:

building the system configuration...
error: The option `system.build.installBootLoader' is defined multiple times.
       Definition values:
       - In `nixos/modules/system/boot/loader/grub/grub.nix': <derivation /nix/store/v1yj7wffg4q1mqk8rahsq5pgvq1jagn8-install-grub.sh.drv>
       - In `nixos/modules/system/boot/loader/systemd-boot/systemd-boot.nix': <derivation /nix/store/kzmd23ma820f3kxx6sylnkc2yfa348cm-systemd-boot.drv>

New error:

nix-repl> (nixos { fileSystems."/".device = "x"; boot.loader.grub.devices = ["/dev/x"]; boot.loader.systemd-boot.enable = true; boot.loader.grub.enable = true; }).config.system.build.installBootLoader
error: The option `system.build.installBootLoader' is defined multiple times.
       Only one bootloader can be enabled at a time. This requirement has not
       been checked until NixOS 22.05. Earlier versions defaulted to the last
       definition. Change your configuration to enable only one bootloader.

       Definition values:
       - In `nixos/modules/system/boot/loader/grub/grub.nix': <derivation /nix/store/v1yj7wffg4q1mqk8rahsq5pgvq1jagn8-install-grub.sh.drv>
       - In `nixos/modules/system/boot/loader/systemd-boot/systemd-boot.nix': <derivation /nix/store/kzmd23ma820f3kxx6sylnkc2yfa348cm-systemd-boot.drv>

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • manually checked nixosTests.nginx drv hash unchanged, manually tested the error
  • automated
    • 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/)
• 22.05 Release Notes (or backporting 21.11 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
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[ncfavier]

@ofborg test grub systemd-boot.basic

[lopsided98]

Previously it was only possible to override an attr in system.build using mkAfter:

system.build = mkAfter {
  attr = ...;
};

This now fails with lib/types.nix:569:38 called with unexpected argument 'priority'. The correct way is now to use mkForce like any other option.

I don't think this breakage needs to be fixed, since the new config is definitely better than the old mkAfter hack. I'm just posting this to help anyone else who runs into this cryptic error message.