Unicode bug

I went with `env = {str(key): str(value) for key, value in env.items()}` as it is robust to any future additions to the `env` dictionary.
If you're going to get rid of this, which I agree is hacky, you should replace all instances of `str` with `conda.compat.text_type` in this module.  There are quite a few places where I assumed everything would be unicode when I wrote this.