there is a build librashader script in the repository, so calling that from the flatpak build script and copying the resulting .so file somewhere ares can find it should be enough.

Note that librashader is rust based, so it will require a working rust toolchain

You can see how our CI does it for windows here https://github.com/ares-emulator/ares/blob/facd8afea47a770980e8abaf0e4f4f57c2ed6767/.github/workflows/build.yml#L93