fix(build): Make repository callable in Nix

This is required for some things that expect package sets to be
callable, e.g. Nixery.

default.nix

@@ -5,6 +5,8 @@
 # This makes packages accessible via the Nixery instance that is configured to
 # use this repository as its nixpkgs source.
 
+{ ... }@args:
+
 with builtins;
 
 let