nixos-rebuild-ng: make --diff works with 'dry-activate' (#493808)

    if args.no_build_nix:

        parser_warn("--no-build-nix is deprecated, we do not build nix anymore")

    if args.diff and args.action not in (

        # case for calling build_and_activate_system

        # except excluding DRY_BUILD and DRY_ACTIVATE,

        # in which --diff is uniquely a no-op

        Action.SWITCH.value,

        Action.BOOT.value,

        Action.TEST.value,

        Action.BUILD.value,

        Action.BUILD_IMAGE.value,

        Action.BUILD_VM.value,

        Action.BUILD_VM_WITH_BOOTLOADER.value,

    if (

        args.action

        in (

            Action.DRY_BUILD.value,  # --diff breaks dry-build

            Action.EDIT.value,

            Action.LIST_GENERATIONS.value,

            Action.REPL.value,

        )

        and args.diff

    ):

        parser_warn(f"--diff is a no-op with '{args.action}'")

        args.diff = False

    if args.action == Action.EDIT.value and (args.file or args.attr):

        parser.error("--file and --attr are not supported with 'edit'")

        parser.error(f"--file and --attr are not supported with '{args.action}'")

    if (args.target_host or args.build_host) and args.action not in (

        Action.SWITCH.value,