`flet_rive` package cannot be built on Linux with the latest version of `rive`

Error:

```
/home/appveyor/projects/flet/client/linux/flutter/ephemeral/.plugin_symlinks/rive_common/macos/SheenBidi/Source/SBParagraph.c:619:16: error: unused variable 'stringLength' [-Werror,-Wunused-variable]
```

Upstream issue: https://github.com/rive-app/rive-flutter/issues/390