nfs-utils: set /etc/krb5.keytab as default path for rpc-gssd

Currently the `rpc-gssd.service` has a `ConditionPathExists` clause that can
never be met, because it's looking for stateful data inside `/nix/store`.

`auth-rpcgss-module.service` also only starts if this file exists.

Fixes NixOS/nixpkgs#29509.

pkgs/os-specific/linux/nfs-utils/default.nix

@@ -39,7 +39,7 @@ in stdenv.mkDerivation rec {
       sed -i "s,^PATH=.*,PATH=$out/bin:${statdPath}," utils/statd/start-statd
 
       configureFlags="--with-start-statd=$out/bin/start-statd $configureFlags"
-      
+
       substituteInPlace systemd/nfs-utils.service \
         --replace "/bin/true" "${coreutils}/bin/true"