pmb.chroot.root: set PYTHONUNBUFFERED=1 (MR 2209)

Set the env var, so python programs running inside pmbootstrap chroot
don't buffer their output and only end up printing everything when they
are done. I've seen this with meson. This is bad for usability because
we don't see output, but also a problem because pmbootstrap kills
commands if they don't print any output for some time (default: 15 min).

pmb/chroot/root.py

@@ -59,6 +59,7 @@ def root(args, cmd, suffix="native", working_dir="/", output="log",
                "HOME": "/root",
                "LANG": "UTF-8",
                "PATH": pmb.config.chroot_path,
+               "PYTHONUNBUFFERED": "1",
                "SHELL": "/bin/ash",
                "TERM": "xterm"}
     for key, value in env.items():