net: don't run TestDialListenerAddr in short mode on non-builders

It listens on all addresses, which users might not want.

Updates #18806 (follow-up to feedback from CL 45088)

Change-Id: I51de2d3fc3cd88a61eb3c63018c47aea920c0549
Reviewed-on: https://go-review.googlesource.com/45157
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>

src/net/dial_test.go

@@ -893,6 +893,9 @@ func TestCancelAfterDial(t *testing.T) {
 // if the machine has halfway configured IPv6 such that it can bind on
 // "::" not connect back to that same address.
 func TestDialListenerAddr(t *testing.T) {
+	if testenv.Builder() == "" {
+		testenv.MustHaveExternalNetwork(t)
+	}
 	ln, err := Listen("tcp", ":0")
 	if err != nil {
 		t.Fatal(err)