Update validate pyenv setup to look at PYENV_ROOT environment variable

Currently, Habushu validates the pyenv setup by checking that `PATH` contains the the following `shimsPath`. 
```
String shimsPath = HabushuUtil.getHomeDirectory() + "/.pyenv/shims";
```
Instead of assuming that pyenv will live in `/.pyenv/shims` (which is the default value), we should check the `PYENV_ROOT` environment variable directly