meta(nix): Derive attribute values of outputs

This makes it possible to programmatically use all the outputs in a
flake without having to manually specify each one individually, useful
for having one main nix-config that can be used with other flakes and
system configurations.

overlays/default.nix

@@ -1,10 +1,5 @@
 { self, ... }:
 
 {
-  nixpkgs.overlays = with self.overlays; [
-    base16-schemes
-    kitty
-    srb2
-    zola
-  ];
+  nixpkgs.overlays = builtins.attrValues self.overlays;
 }