ssh/agent: fix another test to not require IPv4.

Missed a copy/paste of netPipe in change 36110.

Change-Id: I1a850dd9273d71fadc0519cf4cb2a2de6ecae4c2
Reviewed-on: https://go-review.googlesource.com/36259
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>

ssh/agent/client_test.go

@@ -180,7 +180,7 @@ func TestCert(t *testing.T) {
 // therefore is buffered (net.Pipe deadlocks if both sides start with
 // a write.)
 func netPipe() (net.Conn, net.Conn, error) {
-	listener, err := net.Listen("tcp", "127.0.0.1:0")
+	listener, err := net.Listen("tcp", ":0")
 	if err != nil {
 		return nil, nil, err
 	}