Export HOME in pmb.chroot.root() and -.user()

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!

pmb/chroot/root.py

@@ -70,6 +70,7 @@ def root(args, cmd, suffix="native", working_dir="/", output="log",
     # Merge env with defaults into env_all
     env_all = {"CHARSET": "UTF-8",
                "HISTFILE": "~/.ash_history",
+               "HOME": "/root",
                "PATH": pmb.config.chroot_path,
                "SHELL": "/bin/ash",
                "TERM": "xterm"}