nixos/fwupd: Don't use DynamicUser

Was introduced in dde6a4f397532bbd01b346466ea751e7c3cd9d06
but it doesn't work on my machine:

fwupdmgr[439074]: Failed to connect to daemon: The connection is closed

Creating a persistent user does work however

nixos/modules/services/hardware/fwupd.nix

···
       # fwupd-refresh expects a user that we do not create, so just run with DynamicUser
       # instead and ensure we take ownership of /var/lib/fwupd
       services.fwupd-refresh.serviceConfig = {
-        DynamicUser = true;
         StateDirectory = "fwupd";
       };
 
       timers.fwupd-refresh.wantedBy = [ "timers.target" ];
     };
+
+    users.users.fwupd-refresh = {
+      isSystemUser = true;
+      group = "fwupd-refresh";
+    };
+    users.groups.fwupd-refresh = {};
 
     security.polkit.enable = true;
   };