Convert build mode to lowercase in tool_backend (#40393)

On macOS and iOS, the build mode is converted to lowercase in the
Flutter-side script, allowing projects to pass configuration names
directly. This adds the same behavior to the tool_backend.dart script
currently used by Windows and Linux, so that Windows builds can do the
same (since the default configuration names used by VS, as in Xcode,
start with a capital letter).

packages/flutter_tools/bin/tool_backend.dart

@@ -8,7 +8,7 @@ import 'package:path/path.dart' as path; // ignore: package_path_import.
 /// Executes the required flutter tasks for a desktop build.
 Future<void> main(List<String> arguments) async {
   final String targetPlatform = arguments[0];
-  final String buildMode = arguments[1];
+  final String buildMode = arguments[1].toLowerCase();
 
   final String projectDirectory = Platform.environment['PROJECT_DIR'];
   final bool verbose = Platform.environment['VERBOSE_SCRIPT_LOGGING'] != null;