cache libwebsockets in CI jobs

The Ubuntu/Debian package for [libwebsockets](https://libwebsockets.org/) does not install cmake files, so in CI jobs we [build and install it from source code](https://github.com/rerobots/simboard/blob/5438d4d295ccc7ca323f4cec7135edac9b579c9d/.github/workflows/main.yml#L17). To have faster jobs and use less bandwidth, this should be cached.