rust-hypervisor-firmware: make assert lazy (#426900)

in

assert lib.assertMsg (builtins.pathExists target) "Target spec not found";

let

  cross = import ../../../.. {

    system = stdenv.hostPlatform.system;

    crossSystem = lib.systems.examples."${arch}-embedded" // {

      rust.rustcTarget = "${arch}-unknown-none";

      rust.platform = lib.importJSON target;

      rust.platform =

        assert lib.assertMsg (builtins.pathExists target) "Target spec not found";

        lib.importJSON target;

    };

  };