test suite: liveusb-creator hangs in the middle

The “Upgrading an old Tails USB installation from another Tails USB drive” scenario fails for me (bare metal, test/rjb-migration, today’s build of devel branch): Tails Installer is stuck at 61%.

Parent Task: #5978 (closed)

Original created by @intrigeri on 6539 (Redmine)

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