Use the `USER` env var instead of `whoami`

Calling `whoami` will always return the user with which the server was
started, which might or might not be the current user.

For example if we change the user in the middle of a session the prompt
will continue showing the original username.
Merge pull request #7 from nb/use-user-env-var

Use the `USER` env var instead of `whoami`