Change default directory

It would be great if it is possible to change the default directory. As my home directory is write protected, ```buck2``` fails to execute. An investigation shows that ```buck2``` attempts to create ```~/.buck2```.

One solution addressing this is an environment variable to change the default directory. Another potential solution is to follow the [XDG Specification](https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html).