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.
hmm. Not really the same thing

```powershell
PS C:\Users\Administrator\AppData\Roaming\Juju> echo $env:HOMEDRIVE$env:HOMEPATH
C:\Users\Administrator
PS C:\Users\Administrator\AppData\Roaming\Juju> echo $env:APPDATA
C:\Users\Administrator\AppData\Roaming
PS C:\Users\Administrator\AppData\Roaming\Juju>
```
Any reason to change this?