mill: use finalAttrs

[TomaSajt]

Description of changes

The change in this PR would allow for a much easier method of overriding of the mill derivation's version.

Previously you'd have to the following to override the mill version:

mill.overrideAttrs rec {
  version = "0.11.7";
  src = fetchurl {
    url = "https://github.com/com-lihaoyi/mill/releases/download/${version}/${version}-assembly";
    hash = "sha256-iijKZlQoiIWos+Kdq9hIgiM5yM7xCf11abrJ71LO9jA=";
  };
};

After this PR you'll be able to do

mill.overrideAttrs (oldAttrs: {
  version = "0.11.7";
  src = oldAttrs.src.overrideAttrs {
    outputHash = "sha256-iijKZlQoiIWos+Kdq9hIgiM5yM7xCf11abrJ71LO9jA=";
  };
})

---

Note: overriding/pinning the version of mill is useful if you want to create a FOD that contains files fetched by mill. This is because different versions of mill will get different files fetched, thus changing the FOD hash.

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/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 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.

---

Add a 👍 reaction to pull requests you find important.

[DontEatOreo]

LGTM!