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nix: update nixpkgs, remove zig.hook, and remove x11-gnome master

As of NixOS/nixpkgs#473413[1], `zig.hook` no longer supports `zig_default_flags`, and now they can and must be provided in `zigBuildFlags` instead. Updating also requires removing gnome-xorg since it has been removed from nixpkgs. [1] https://github.com/NixOS/nixpkgs/pull/473413