Update zkapauthorize to use mach-nix and nixos-21.05.

nixos/pkgs/default.nix

@@ -20,6 +20,7 @@ let
 in
 {
   zkapissuer = callPackage ./zkapissuer {};
+  inherit (ourpkgs) leasereport;
   zkap-spending-service = callPackage ./zkap-spending-service {};
-  inherit (ourpkgs) privatestorage leasereport;
+  privatestorage = callPackage ./privatestorage.nix {};
 }