pmb.chroot.root: set PYTHONUNBUFFERED=1
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).