Set HOME to /root for commands started with pmb.chroot.root() and to
/home/pmos for commands started with pmb.chroot.user().
POSIX requires this variable to be set, see:
<http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html>
And this prevents a crash in "adb", which takes and alternative code
path if HOME is not set, that does not work with musl (fixes#1638).
Thanks to @ryang2678 for figuring this out!