Adopt the Nix packaging from our nixpkgs fork and use it

nixos/modules/issuer.nix

@@ -2,8 +2,7 @@
 # ZKAPs.
 { lib, pkgs, config, ... }: let
   cfg = config.services.private-storage-issuer;
-  # Our own nixpkgs fork:
-  ourpkgs = import ../../nixpkgs-ps.nix {};
+  zkapissuer = import ../pkgs/zkapissuer {};
 in {
   imports = [
     # Give it a good SSH configuration.
@@ -13,7 +12,7 @@ in {
   options = {
     services.private-storage-issuer.enable = lib.mkEnableOption "PrivateStorage ZKAP Issuer Service";
     services.private-storage-issuer.package = lib.mkOption {
-      default = ourpkgs.zkapissuer.components.exes."PaymentServer-exe";
+      default = zkapissuer.components.exes."PaymentServer-exe";
       type = lib.types.package;
       example = lib.literalExample "pkgs.zkapissuer.components.exes.\"PaymentServer-exe\"";
       description = ''

nixos/pkgs/zkapissuer/repo.nix

+{ fetchFromGitHub }:
+fetchFromGitHub {
+  owner = "PrivateStorageio";
+  repo = "PaymentServer";
+  rev = "2ca189c51a5314eb476d1866d28ee483701af6f5";
+  sha256 = "1sb53lvi6alwdjhx4cv1gg7ywkqf18n1zjgdj3nigq3vjwxhdcpb";
+}

nixos/pkgs/zkapissuer/zkapissuer.nix

+{ callPackage }:
+let
+  repo = callPackage ./repo.nix { };
+in
+  (import "${repo}/nix").PaymentServer