WiP: fix environs tests on windows

Unfortunately, permission checking on Windows does not work as expected. For example, doing a:

```go
os.Chmod("C:/test.txt", 0600)
```

will set permissions to:

```
-rw-rw-rw-
```
We have a utility for this, please update the test to use it:
http://godoc.org/github.com/juju/utils/tar#UntarFiles