How to reference OS Environment Variables in nginx.conf

In nginx.conf.

After set a variable by set $name value, i can reference it like $name,

But when I export an OS Environment Variable by env name_from_env, like https://nginx.org/en/docs/ngx_core_module.html#env said, and i am sure the name_from_env is valid which defined form nginx's parent process.

But, my friends, how to reference it ? $name_from_env or ${name_from_env} or %name_from_env% didn't work what I've tried before.