ipn/ipnlocal: fix AllowsUpdate disable after enable (#9827)

The old code would always retain value `true` if it was set once, even
if you then change `prefs.AutoUpdate.Apply` to `false`.
Instead of using the previous value, use the default (envknob) value to
OR with.

Updates #755

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

ipn/ipnlocal/local.go

@@ -3793,7 +3793,7 @@ func (b *LocalBackend) applyPrefsToHostinfoLocked(hi *tailcfg.Hostinfo, prefs ip
 	hi.RoutableIPs = prefs.AdvertiseRoutes().AsSlice()
 	hi.RequestTags = prefs.AdvertiseTags().AsSlice()
 	hi.ShieldsUp = prefs.ShieldsUp()
-	hi.AllowsUpdate = hi.AllowsUpdate || prefs.AutoUpdate().Apply
+	hi.AllowsUpdate = envknob.AllowsRemoteUpdate() || prefs.AutoUpdate().Apply
 
 	var sshHostKeys []string
 	if prefs.RunSSH() && envknob.CanSSHD() {