As of NixOS/nixpkgs#473413, `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.
`nix flake check` succeeds on my system (x86_64-linux), with a couple
deprecation warnings that I believe aren't important.
This is stomping towards minimizing our build.zig dependencies so that
it can be cached more often. Right now, touching almost any file in the
project forces the build.zig to rebuild which is destroying my
productivity.
