If $HOME doesn't exist, don't create it with 0777 permissions

[iamed2]

Experienced during #1144

I recognize this is app_dirs2 defaults taking effect, but perhaps this is something where the defaults should be avoided or worked around (e.g. bail if app_dirs2::get_app_root returns a directory that doesn't exist). It is unexpected for a document generation program to create a globally-writeable home directory, which may silently reduce security by allowing any other program running as any user to manipulate the behaviour of programs running as the user that ran Tectonic.

Implementing #1144 would allow people to avoid this situation in other ways.