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 different path than that of utils.Home() (which is much more widely used along the project, if not everywhere now).

Now JujuHome() will return a path 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.