https://github.com/tailscale/tailscale/pull/15395 changed the logic to
skip `EditPrefs` when the platform doesn't support auto-updates. But the
old logic would only fail `EditPrefs` if the auto-update value was
`true`. If it was `false`, `EditPrefs` would succeed and store `false`
in prefs. The new logic will keep the value `unset` even if the tailnet
default is `false`.
Fixes#15691
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>