Update nixos to 21.11

This also update ZKAPAuthorizer and zkap-spending-service to versions tested against that release. For ZKAPAuthorizer, this includes a switch to using python3.

morph/lib/default.nix

@@ -17,7 +17,7 @@
   # installed, as well as the NixOS module set that is used.
   # This is intended to be used in a grid definition like:
   #     network = { ... ; inherit (gridlib) pkgs; ... }
-  pkgs = import ../../nixpkgs-2105.nix {
+  pkgs = import ../../nixpkgs.nix {
     # Ensure that configuration of the system where this runs
     # doesn't leak into what we build.
     # See https://github.com/NixOS/nixpkgs/issues/62513