Add back Steam Deck and add experimental unstable

overlays/default.nix

@@ -1,8 +1,8 @@
-{ unstable, ... }:
+{ pkgs, ... }:
 {
-  nixpkgs.overlays = [
-    (import ./unstable { inherit unstable; })
-    (final: _prev: import ./packages { pkgs = final; })
-    (import ./mpv)
+  imports = [
+    ./unstable
+    ./packages
+    ./mpv
   ];
 }