handOffWindowsBootstrapRecovery() writes the update marker
unconditionally, same as applyUpdates() before the previous commit.
It's reachable during boot whenever resolveHermesBackend reports
bootstrap-needed, which a relaunch mid-update can plausibly trigger
on Windows -- clobbering a live updater's marker through this second
path. Apply the same updateHandoffConflict() guard here: refuse to
spawn a second updater when one is already alive, and quit instead
so the live updater can finish and restart us.
64c1db961f