Replace deprecated preferred_cli_env with def cli

[stewartj3]

Mix deprecated setting :preferred_cli_env. Move to the new def cli across nx, exla, and torchx.

warning: setting :preferred_cli_env in your mix.exs "def project" is deprecated, set it inside "def cli" instead:

def cli do
  [preferred_envs: [docs: :docs, "hex.publish": :docs]]
end