Gotta call it like a package

nixos/modules/issuer.nix

@@ -2,7 +2,7 @@
 # ZKAPs.
 { lib, pkgs, config, ... }: let
   cfg = config.services.private-storage-issuer;
-  zkapissuer = import ../pkgs/zkapissuer {};
+  zkapissuer = pkgs.callPackage ../pkgs/zkapissuer { };
 in {
   imports = [
     # Give it a good SSH configuration.