cmd/tailscale/cli: fix vet warning

And add vet to the "make check" target, like the CI has.

Makefile

@@ -1,7 +1,10 @@
 usage:
 	echo "See Makefile"
 
-check: staticcheck
+vet:
+	go vet ./...
+
+check: staticcheck vet
 
 staticcheck:
 	go run honnef.co/go/tools/cmd/staticcheck -- $$(go list ./... | grep -v tempfork)

cmd/tailscale/cli/debug.go

@@ -63,5 +63,4 @@ func runMonitor(ctx context.Context) error {
 	mon.Start()
 	log.Printf("Started link change monitor; waiting...")
 	select {}
-	return nil
 }