One-Off Dyno doesn't make CLI user available

Hi, I'd like to know who the authorized CLI user is, for a One-Off Dyno. Hopefully I'm wrong and it's available somehow.

I expect to be able to start a One-Off Dyno and see the user that created it from within the Dyno.

**Example:**

```
heroku run bash -a my_app
echo $HEROKU_ONE_OFF_USER
me@mydomain.com
```

I imagine this could be implemented behind the scenes in the API by passing an ENV, if not already available in another way.