nixos-rebuild-ng: add FLAKE_FLAGS to nix copy

pkgs/by-name/ni/nixos-rebuild-ng/src/nixos_rebuild/nix.py

@@ -210,6 +210,7 @@ def copy_closure(
         run_wrapper(
             [
                 "nix",
+                *FLAKE_FLAGS,
                 "copy",
                 *dict_to_flags(copy_flags),
                 "--from",

pkgs/by-name/ni/nixos-rebuild-ng/src/tests/test_main.py

@@ -688,6 +688,8 @@ def test_execute_nix_switch_build_target_host(
             call(
                 [
                     "nix",
+                    "--extra-experimental-features",
+                    "nix-command flakes",
                     "copy",
                     "--from",
                     "ssh://user@build-host",

pkgs/by-name/ni/nixos-rebuild-ng/src/tests/test_nix.py

@@ -263,6 +263,8 @@ def test_copy_closure(monkeypatch: MonkeyPatch) -> None:
         mock_run.assert_called_with(
             [
                 "nix",
+                "--extra-experimental-features",
+                "nix-command flakes",
                 "copy",
                 "--copy-flag",
                 "--from",