template: Use `flatpak-builder` instead of `flatpak build`
flatpak build
does not support apps that use an SDK like
the Rust SDK.
This allows apps like Fractal that take advantage of SDKs to use the CI templates.
flatpak build
does not support apps that use an SDK like
the Rust SDK.
This allows apps like Fractal that take advantage of SDKs to use the CI templates.