fix(flake): guard linux-only deps behind stdenv.isLinux

wayland, alsa-lib, and udev are Linux-only — including them
unconditionally breaks evaluation on darwin.

flake.nix

@@ -20,16 +20,18 @@
       let
         overlays = [ (import rust-overlay) ];
         pkgs = import nixpkgs { inherit system overlays; };
-        runtimeLibs = with pkgs; [
+        runtimeLibs =
+          with pkgs;
+          [
             libx11
             libxcursor
             libxi
             libxkbcommon
             libxcb
-          wayland
             vulkan-loader
             glfw
-        ];
+          ]
+          ++ lib.optional pkgs.stdenv.isLinux wayland;
 
         takeScreenshots = pkgs.writeShellApplication {
           name = "take-screenshots";
@@ -135,6 +137,8 @@
                 ];
               })
               pkg-config
+            ]
+            ++ lib.optionals pkgs.stdenv.isLinux [
               alsa-lib.dev
               udev.dev
             ]