I have a serverless application defined like this

```
service: hello_world

frameworkVersion: '3'

provider:
  name: aws
  runtime: python3.8

functions:
  hello:
    handler: handler.run
  world:
    handler: handler2.run
    url: true

```

Part of the function `hello` is to hand the function url of `world` to an external api.

How can I get the function url to `hello`, as an environment variable or otherwise?