ggml : allow setting the backend name with env variable

[rgerganov]

ref: #1058

[slaren]

I am not sure that this is the best way to implement this, because ggml_backend_load_best only looks in a list of pre-defined paths that may not be entirely obvious to the user, and this behavior may change in the future. It may make more sense to use ggml_backend_load with the full file path in the environment variable.