Add shell.nix instead of flake.nix

shell.nix

@@ -0,0 +1,20 @@
+{pkgs ? import <nixpkgs> {}}:
+pkgs.mkShell {
+  packages = with pkgs; [
+    zig_0_12
+    wlroots_0_17
+    libGL
+    libevdev
+    libinput
+    libxkbcommon
+    pixman
+    udev
+    wayland-protocols
+    xorg.libX11
+    pkg-config
+    wayland
+    xwayland
+    valgrind
+    gdb
+  ];
+}