prompt_install_mode guarded its prompt with `[ ! -r /dev/tty ]`, which
never fires on Linux: /dev/tty is mode 0666 whether or not the process
has a controlling terminal, and only opening it fails. Without a
terminal the menu was printed, the read failed with "No such device or
address", and the default was taken by accident rather than by the
documented path.
The guard now opens /dev/tty in a subshell and takes the "no terminal
for a prompt" path when that fails.
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