flake.nix: add derivation warning for maintainerless note

We can show a warning for all derivations that don't have maintainers,
keeping it commented because they will throw a warning for every time
they are evaluated.

flake.nix

@@ -150,6 +150,7 @@
       channels-config = {
         # allowBroken = true;
         allowUnfree = true;
+        # showDerivationWarnings = [ "maintainerless" ];
 
         # TODO: cleanup when available
         permittedInsecurePackages = [