add zig overlay

Unfortunately, this is not working because of an issue with graphical
dependencies.

flake.nix

@@ -2,8 +2,9 @@
   description = "FabApp";
 
   inputs = {
-    # take unstable if not specified
-    # nixpkgs.url = "github:nixos/nixpkgs/release-23.05";
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-23.05";
+    nixpkgs-unstable.url = "github:nixos/nixpkgs/nixpkgs-unstable";
+    zig-overlay.url = "github:mitchellh/zig-overlay";
     flake-parts.url = "github:hercules-ci/flake-parts";
   };
 
@@ -17,11 +18,18 @@
 
       # https://flake.parts/module-arguments.html#persystem-module-parameters
       perSystem = { pkgs, system, ... }: {
-        _module.args.pkgs = import nixpkgs {
+        # We need unstable for Zig
+        _module.args.pkgs = import inputs.nixpkgs-unstable {
           inherit system;
-          # config.allowUnfree = true;
+          # https://flake.parts/overlays
+          overlays = [
+            inputs.zig-overlay.overlays.default
+          ];
+          config = { };
         };
 
+        formatter = pkgs.nixfmt;
+
         devShells.default = with pkgs;
           mkShell {
             buildInputs = [
@@ -29,20 +37,23 @@
 
               # Zig / webAssembly impl.
               zig
+              # zigpkgs."master" # Issue with X11/GL libraries
               zls
               emscripten
 
-              # Tools
-              nixfmt
-
               # Linux graphical deps
               xorg.libX11
               libGL
+              # xorg.libXcursor
+              # xorg.libXi
+              # xorg.libXext
+              # xorg.libXrandr
+              # xorg.libXinerama
+              # libGLU
             ];
 
           };
 
-        formatter = pkgs.nixfmt;
       };
     };
 }