nixos-rebuild-ng: make --diff works with 'dry-activate'

Also make it actually a no-op with dry-build, since it breaks this
specific command.

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

@@ -265,22 +265,12 @@ def parse_args(
     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 == Action.DRY_BUILD.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,