refactor(flake): replace flake-utils with nix-systems

[luisnquin]

Overview

This change removes the dependency on flake-utils and replaces it with nix-systems.
It introduces a forAllSystems helper based on nixpkgs.lib.genAttrs to generate development shells for all supported systems.
The overall structure of the flake remains the same, but the dependency graph is simplified and more explicit.

Type of change

• Created a new plugin
• Improved an existing plugin
• Fixed a bug in an existing plugin
• Improved contributor utilities or experience

Related Issue(s)

• Resolves: #
• Relates: #

How To Test

1. Run nix develop on any supported system.
2. Confirm that a development shell is created with the Go toolchain (go, gopls).
3. Validate that nixosModules and hmModules can still be imported as before.

Changelog

Replaced flake-utils with nix-systems and refactored flake outputs to generate development shells for all supported systems.

[luisnquin]

CI is just failing on the old nixpkgs pin, not from this change.

[mrjones2014]

Hi, thanks for your contribution!

If you rebase on main your CI should pass, some long-needed maintenance surrounding the nix stuff has finally merged.

[mrjones2014]

I'm happy to approve once that's done, this change looks good.

[luisnquin]

This looks ready. :)

[mrjones2014]

Looks great and cuts down a bit of bloat in the dep tree! Thanks!

[mrjones2014]

Maybe @volodymyrZotov can provide a second approval?