cmd/tailscale/cli: use errors.Is to check ff's wrapped flag errors

And also check from its Parse method.

Change-Id: I18754920575254cb6858a16b7954e74aa16483a1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/cli.go

@@ -139,13 +139,16 @@ change in the future.
 	}
 
 	if err := rootCmd.Parse(args); err != nil {
+		if errors.Is(err, flag.ErrHelp) {
+			return nil
+		}
 		return err
 	}
 
 	tailscale.TailscaledSocket = rootArgs.socket
 
 	err := rootCmd.Run(context.Background())
-	if err == flag.ErrHelp {
+	if errors.Is(err, flag.ErrHelp) {
 		return nil
 	}
 	return err