Fix the ^[[13;1R cursor-position-report artifacts visible next to the
first configurator step. archiso's boot init leaves DSR responses in the
kernel tty input buffer; the first gum invocation reads them as stray
input and echoes them as visible escape sequences.
- Drain stdin (with a short timeout) from /dev/tty before configurator
runs so any pending replies are consumed.
- Drop the explicit </dev/tty stdin redirect — the parent script's
stdin is already tty1 (inherited from archiso autostart), and the
explicit redirect opened a new fd with subtly different termios.