maintainers/scripts/update.nix: derivation is the final station

It does not make sense to look for derivations within derivations,
not even when `recurseForDerivations` is true. Nix does not do that either:

https://github.com/NixOS/nix/blob/ebc024df2287085d48ed6194aa756fd70c07f76c/src/libexpr/get-drvs.cc#L346-L355

maintainers/scripts/update.nix

@@ -32,8 +32,8 @@ let
           (name: pkg:
             let
               result = builtins.tryEval (
-                if lib.isDerivation pkg && cond name pkg
+                if lib.isDerivation pkg
-                  then [(return name pkg)]
+                  then lib.optional (cond name pkg) (return name pkg)
                 else if pkg.recurseForDerivations or false || pkg.recurseForRelease or false
                   then packagesWith cond return pkg
                 else []