Fix Windows home directory error

[danepowell]

Based on downstream user reports (acquia/cli#1611), there's a bug in home directory detection on Windows.

It seems like Terminus doesn't officially support native Windows terminals (CMD, PowerShell, etc), in which case maybe this is dead code that should be removed entirely.

[namespacebrian]

Correct, running terminus on Windows is only officially supported via Windows Subsystem for Linux. Trying to provide support for Windows environments (documenting setup, troubleshooting users' problems, etc) is too much for us to do reliably.

I don't think we want to actively prevent people from maybe using it on Windows, we just can't afford to track down every problem, or gate fixes / new features based on Windows compatibility.. and since we're not doing that, it's probably safe to assume that if it works on (non-WSL) Windows at all right now, we can't trust that it will continue to work in the future.

This change seems small enough that maybe we could merge it. If I'm reading the acquia/cli issue correctly, getenv is returning a boolean. The existing terminus code effectively assumes it's going to return null or a string. Switching to truthy/falsey seems reasonable to me. Or maybe doing one better and checking if it's a string of non-zero length.. since the code seems to assume it's going to be a string...