Function detect_backend 

pub async fn detect_backend() -> Result<Arc<dyn BuildBackend>>

Auto-detect the best available build backend for the current platform.

Selection order:

1. If the ZLAYER_BACKEND environment variable is set to "buildah" or "sandbox", that backend is forced.
2. On macOS, buildah is tried first; if unavailable, the sandbox backend is returned as a fallback.
3. On Linux, only the buildah backend is available.

Errors

Returns an error if no usable backend can be found.