nixos/systemd: add to system.nssDatabases.group too

nixos/modules/config/nsswitch.nix uses `passwdArray` for both `passwd`
and `group`, but when moving this into the systemd module in
c0995d22eed1a19ac9442c8460c18dd6a4c389b7, it didn't get split
appropriately.

nixos/modules/system/boot/systemd.nix

@@ -848,6 +848,10 @@ in
         [ "mymachines" ]
         (mkAfter [ "systemd" ])
       ]);
+      group = (mkMerge [
+        [ "mymachines" ]
+        (mkAfter [ "systemd" ])
+      ]);
     };
 
     environment.systemPackages = [ systemd ];