Make run-pass/env-home-dir.rs test more robust
Make run-pass/env-home-dir.rs test more robust

Remove the assumption that home_dir always returns Some

This allows the test to be executed with
[cross](https://github.com/japaric/cross).

GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust