unmount of /sys when tearing down mountpoint can fail

[Michael Hudson-Doyle]

From https://launchpad.net/~ubuntu-cdimage/+livefs/ubuntu/mantic/cpc/+build/488861:

[10] install_packages
Error: Error running command "/usr/bin/umount /tmp/ubuntu-image-bd37159b-17f8-497d-9487-ee43b422ae5f/chroot/sys". Error is "exit status 32". Output is:
umount: /tmp/ubuntu-image-bd37159b-17f8-497d-9487-ee43b422ae5f/chroot/sys: target is busy.

From debugging the isomorphic problem in live-build, what has happened here is very likely that a submount under /sys has appeared between /sys being bindmounted into the chroot and the attempt at unmount. I have already written an epic comment about what is likely going on here when we had the same problem with curtin: https://git.launchpad.net/curtin/tree/curtin/util.py#n532. Probably ubuntu-image can use --make-rprivate and umount --recursive though.

[Łukasz Zemczak (sil2100)]

I think with PR https://github.com/canonical/ubuntu-image/pull/139 landed, this should now be better.