provider/dns: update hard-coded DNS result in acctests

The acctests depend on getting a particular result back from resolving
www.hashicorp.com, which seems to have changed. In the long run we should
probably *not* be depending on the DNS configuration of the main Hashicorp
website, but this is just a quick fix to get the build back to green so
we can continue work on other things.

builtin/providers/dns/data_dns_cname_record_set_test.go

@@ -17,7 +17,7 @@ func TestAccDnsCnameRecordSet_Basic(t *testing.T) {
 			  host = "www.hashicorp.com"
 			}
 			`,
-			"s.shared.global.fastly.net.",
+			"dualstack.s.shared.global.fastly.net.",
 		},
 	}