nix: use zig2nix to manage nix cache/deps

This brings the internal package more in line with how the nixpkgs
package is built. It also handles recursive dependencies better than the
current system.

nix/devShell.nix

@@ -55,6 +55,8 @@
   wayland,
   wayland-scanner,
   wayland-protocols,
+  zig2nix,
+  system,
 }: let
   # See package.nix. Keep in sync.
   rpathLibs =
@@ -102,6 +104,7 @@ in
         scdoc
         zig
         zip
+        zig2nix.packages.${system}.zon2nix
 
         # For web and wasm stuff
         nodejs