run_completion runs in the OLD process after the checkout swap, and its
contract is never to import application helpers from the replacement tree.
Importing pm.libatomic there crashed the update when the new tree lacked
the module (downgrade or older pin) and on architectures where
current_target() raises. Run the pre-install as its own process from the
new tree, ignore its outcome, and let the in-lock repair report.
Also: gate on an openable /dev/tty first (cheapest check, and the detached
child exits immediately), treat an unsupported architecture as "skip",
bound the sudo -n probe, and drop the timeout on the interactive install
so a slow prompt or mirror is never killed mid-dpkg (it runs outside PM's
lock). Found in the final-head review of #132664.
47e6c2ebb5