cmd/tailscale/cli: create netmon in debug ts2021

Otherwise we'll see a panic if we hit the dnsfallback code and try to
call NewDialer with a nil NetMon.

Updates #14161

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I81c6e72376599b341cb58c37134c2a948b97cf5f

control/controlhttp/constants.go

@@ -76,6 +76,8 @@ type Dialer struct {
 	// dropped.
 	Logf logger.Logf
 
+	// NetMon is the [netmon.Monitor] to use for this Dialer. It must be
+	// non-nil.
 	NetMon *netmon.Monitor
 
 	// HealthTracker, if non-nil, is the health tracker to use.