cockpit: specify --with-admin-group

Until PR #236458 it was doing auto-detection (!)
which chose "root" (at least sometimes):
https://hydra.nixos.org/log/wbxd753q70plgxjm3xdljky9a81692kk-cockpit-287.drv
Now it failed due to missing getent:
https://hydra.nixos.org/build/225207182/nixlog/2/tail

pkgs/servers/monitoring/cockpit/default.nix

@@ -149,6 +149,7 @@ stdenv.mkDerivation rec {
     "--enable-prefix-only=yes"
     "--disable-pcp" # TODO: figure out how to package its dependency
     "--with-default-session-path=/run/wrappers/bin:/run/current-system/sw/bin"
+    "--with-admin-group=root" # TODO: really? Maybe "wheel"?
   ];
 
   enableParallelBuilding = true;