top-level: ignore unexpected args

This fixes both `nix-shell` failing to eval with `nixUnstable`, as well
as ofborg's failure to eval on aarch64 due to passing an "unexpected
arg" (1112e3a8c8/ofborg/src/nix.rs (L334-L340)).

pkgs/top-level/default.nix

@@ -38,6 +38,9 @@
   # environment. See below for the arguments given to that function, the type of
   # list it returns.
   stdenvStages ? import ../stdenv
+
+, # Ignore unexpected args.
+  ...
 } @ args:
 
 let # Rename the function arguments