systemdUkify: fixup build

This is basically a revert of a tiny part of PR #429972.

pkgs/os-specific/linux/systemd/default.nix

@@ -905,7 +905,7 @@ stdenv.mkDerivation (finalAttrs: {
         )
       );
 
-  disallowedRequisites = [
+  disallowedRequisites = lib.optionals (!withUkify) [
     bash
     bashNonInteractive
   ];