environs: fixes for Windows

This PR makes some transparent changes to implement filestorage on windows.

We cannot write files with : on the disk so we write everything with a -. This also means that anything that's read from the disk also has to contain dashes.

Depends on https://github.com/juju/utils/pull/45
Kind of a derp. I messed around with filepath.Join inside the if at some point and I didn't want file:\\(...)\\\ so I just switched them around and forgot them like that