Update file permissions for FlutterMacOS.framework (flutter#52930)

During FlutterMacOS.framework engine universal fat framework creation, make sure the framework is `u=rwx,go=rx` when it's created.

There's a framework-side workaround in-flight flutter/flutter#148580. 

However, this is still a good thing to do engine-side, particularly for add-to-app when they don't go through the `flutter assemble` tooling.

I'll also add a test around here, once this rolls https://github.com/flutter/flutter/blob/02a6c91e4d37d28f42c8f8e4d4335b0defed41c1/packages/flutter_tools/test/host_cross_arch.shard/macos_content_validation_test.dart#L30

Fixes flutter/flutter#148354

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

sky/tools/create_macos_framework.py

@@ -86,6 +86,8 @@ def main():
   subprocess.check_call([
       'lipo', arm64_dylib, x64_dylib, '-create', '-output', fat_framework_binary
   ])
+  # Make the framework readable and executable: u=rwx,go=rx.
+  subprocess.check_call(['chmod', '755', fat_framework_binary])
 
   # Add group and other readability to all files.
   versions_path = os.path.join(fat_framework, 'Versions')