WiP: Made proper use of utils.Home() on Windows in the juju/osenv package.

Previously, if you called JujuHome() from the juju/osenv package, it would return a path with a different root than that of utils.Home() (which is much more widely used as a base along the project, if not everywhere now).

Now JujuHome() will return a path with a root consistent with the one of utils.Home(), preventing any possible conflicts which might have arisen from this.

The associated tests were also updated, OS-specific skippage being added as well.
Then this is a consistency issue. We should strive to use the same APPDATA dir everywhere. Please note, that application data should be in $env:APPDATA, and not in the users home folder directly.

If juju uses Home() to store such information, then that should be fixed as well.