Function build 

pub fn build<R, E, F, Fut>(
    env: E,
    factory: F,
) -> LocalBoxFuture<'static, Result<()>>
where
    R: Runtime + 'static,
    E: Env<<R as RuntimeDef>::Cli> + Send + 'static,
    F: FnOnce(&mut Context<R>) -> Fut + 'static,
    Fut: Future<Output = Result<R>> + 'static,

Creates a new runtime execution future within a new tokio::task::LocalSet, using a custom environment configuration provider and a runtime factory