Fix for macOS build issue in Flutter >= 3.35
Since version 3.35, the behavior of macos_assemble.sh and xcode_backend.sh
is almost identical. This caused the same error for macOS that previously
occurred for iOS.
Derived from the iOS patch: ./fix-ios-build-xcode-backend-sh.patch
Example error:
```
$ flutter run -d macos
Launching lib/main.dart on macOS in debug mode...
Target debug_unpack_macos failed: Error: Flutter failed to create a directory at "/<nix-store>/XXXX-flutter-3.35.1-unwrapped/bin/cache/artifacts".
Please ensure that the SDK and/or project is installed in a location that has read/write permissions for the current user.
Failed to copy Flutter framework.
** BUILD FAILED **
```
