nixos/jackett: fix hardening for custom dataDir (#390609)

nixos/modules/services/misc/jackett.nix

···
         ProtectProc = "invisible";
         ProtectSystem = "strict";
         ReadWritePaths = [
-          "${config.users.users.${cfg.user}.home}"
+          cfg.dataDir
         ];
         RemoveIPC = true;
         RestrictAddressFamilies = [