nixos/anubis: init module

[soopyc]

Adds a NixOS module for Anubis.

Depends-on: #391924

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/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 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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[isabelroses]

I would love to see this PR merged. Just a few comments from me though.

I see a few uses of builtins but ideally we should use lib for those as they may improve backwards compatibility with older nix versions.

[soopyc]

rebased to master so we can actually run tests
@ofborg test anubis

[Krutonium]

Not weighing in with a review myself as this is generally above my head, but I'd love to see this merged as I've spent far too long recently defending against AI scrapers maxing out my box.