Tails Installer sometimes hangs late in the installation process

"Installing Tails to a pristine USB drive" often stalls with "Unmounting /dev/sda1" at 98%.

Still happens with 0.18~rc1, despite it has the commit fb3d08a5 from upstream liveusb-creator that we hoped would help.

Feature Branch: bugfix/6092-drop-racy-code

Original created by @tails on 6092 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information