This fixes <https://bugs.gnu.org/39712>.
The guix-daemon was preventing the cow-store umount, so restart it. Some
udevd workers, using cow-store files might also still be around, so have some
umount retries.
* gnu/installer/final.scm (kill-cow-users): New procedure,
(umount-cow-store): restart guix-daemon and kill all processes started from
within the cow-store before trying to umount the store overlay. Also try 5
times to umount the overlay in case it is still busy.
64704be417