acme: fix nil pointer dereference in pebble test error reporting

The WaitOrder failure path used errors.Is with a nil *OrderError
target, which never matches and left orderErr nil at the point its
Problem field was dereferenced. Use errors.As to populate the target
instead.

Found by govet's nilness check.

Change-Id: Ieef849fcbf04c85ef32651059db2b5fe162af325
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/803181
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

acme/pebble_test.go

diff --git a/acme/pebble_test.go b/acme/pebble_test.go
index 79051ac..08dbab4 100644
--- a/acme/pebble_test.go
+++ b/acme/pebble_test.go
@@ -383,7 +383,7 @@
 	order, err = client.WaitOrder(ctx, order.URI)
 	if err != nil {
 		var orderErr *acme.OrderError
-		if errors.Is(err, orderErr) {
+		if errors.As(err, &orderErr) {
 			t.Fatalf("failed to wait for order %s: %s: %s", orderURL, err, orderErr.Problem)
 		} else {
 			t.Fatalf("failed to wait for order %s: %s", orderURL, err)