wgengine/magicsoc,net/tstun: put disco key advertisement behind a nob (#18857)

To be less spammy in stable, add a nob that disables the creation and
processing of TSMPDiscoKeyAdvertisements until we have a proper rollout
mechanism.

Updates #12639

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -4309,6 +4309,10 @@ type NewDiscoKeyAvailable struct {
 //
 // We do not need the Conn to be locked, but the endpoint should be.
 func (c *Conn) maybeSendTSMPDiscoAdvert(de *endpoint) {
+	if !buildfeatures.HasCacheNetMap || !envknob.Bool("TS_USE_CACHED_NETMAP") {
+		return
+	}
+
 	de.mu.Lock()
 	defer de.mu.Unlock()
 	if !de.sentDiscoKeyAdvertisement {