Fix User creation rake task

```permissions``` variable is undefined in this context (https://github.com/alphagov/signonotron2/blob/master/lib/tasks/users.rake#L4-L12). Permissions should be passed using environment variable.

Looks like the issue was introduced in https://github.com/alphagov/signonotron2/commit/4474af0b02b8d41c1bc0bd86435b98a983f0958f