keybase service: Turn off debug logging

Keybase is _extremely_ verbose with its debug output when run with -d.

nixos/modules/services/networking/keybase.nix

···
       description = "Keybase service";
       serviceConfig = {
         ExecStart = ''
-          ${pkgs.keybase}/bin/keybase -d service --auto-forked
+          ${pkgs.keybase}/bin/keybase service --auto-forked
         '';
         Restart = "on-failure";
         PrivateTmp = true;