Bug#1053457: live-build: Building live image fails when unmounting /sys: 'target is busy'

[Emanuele Rocca <ema@debian.org>]

Package: live-build
Version: 1:20230502
Severity: normal

Dear Maintainer,

Building a live image with `lb build` fails towards the end with the
following error:

 P: Begin unmounting /sys...
 umount: /tmp/sid-image/chroot/sys: target is busy.
 E: An unexpected failure occurred, exiting...

The issue can be reproduced as follows:

 lb config --distribution sid --updates false --archive-areas 'main non-free-firmware' --bootloaders grub-efi
 echo live-task-lxde > config/package-lists/desktop.list.chroot
 lb build --debug