kyua fails to start with a non-existing homedir

[nbraud]

I'm currently packaging kyua for Debian (as a test dependency of pkgconf), and I noticed that kyua hard aborts when the current homedir does not exist; this is the case in one of the Debian build sandboxes, sbuild, which sets HOME=/sbuild-nonexistent.

This would make things much easier if kyua didn't needlessly abort under such conditions.
Alternatives include:

• not saving logs when the homedir is inexistent/not-writeable;
• using a temporary directory if ~/.kyua isn't available.