netcheck: include DERPPort in the browser HTTPS probe URL

The js/wasm netcheck path probes each DERP region over HTTPS to measure
latency and pick a home relay. It builds that URL from HostName alone, so
a DERP server on a non-443 port is never reachable and the client ends up
with no home DERP.

derphttp's urlString already handles DERPPort; this is the same fix for
the one remaining browser path that does not.

--- a/net/netcheck/netcheck.go
+++ b/net/netcheck/netcheck.go
@@ -1075,7 +1075,11 @@
 		}
 		wg.Go(func() {
 			node := rg.Nodes[0]
-			req, _ := http.NewRequestWithContext(ctx, "HEAD", "https://"+node.HostName+"/derp/probe", nil)
+			host := node.HostName
+			if node.DERPPort != 0 && node.DERPPort != 443 {
+				host = net.JoinHostPort(host, fmt.Sprint(node.DERPPort))
+			}
+			req, _ := http.NewRequestWithContext(ctx, "HEAD", "https://"+host+"/derp/probe", nil)
 			// One warm-up one to get HTTP connection set
 			// up and get a connection from the browser's
 			// pool.