Add FlutterMacOS.xcframework artifact (#143244)

Replace `FlutterMacOS.framework` cached artifact with `FlutterMacOS.xcframework`. Also, update usage of `FlutterMacOS.framework` to use `FlutterMacOS.xcframework`.

Part of https://github.com/flutter/flutter/issues/126016.

dev/devicelab/lib/tasks/plugin_tests.dart

@@ -118,6 +118,12 @@ class PluginTest {
     // Currently this test is only implemented for macOS; it can be extended to
     // others as needed.
     if (buildTarget == 'macos') {
+      // When using a local engine, podhelper.rb will search for a "macos-"
+      // directory within the FlutterMacOS.xcframework, so create a dummy one.
+      Directory(
+        path.join(buildDir.path, 'FlutterMacOS.xcframework/macos-arm64_x86_64'),
+      ).createSync(recursive: true);
+
       // Clean before regenerating the config to ensure that the pod steps run.
       await inDirectory(Directory(app.rootPath), () async {
         await evalFlutter('clean');