I've been working with the pulumi awsx package to set up a VPC and encountered an issue where only the VPC was set with IPv6 cidr block, but not the subnet. I've set the "assignGeneratedIpv6CidrBlock" parameter to true in the awsx.ec2.Vpc config. Is it not possible for the subnet to automatically assigned ipv6 cidr? If the subnet cannot be automatically assigned an IPv6 CIDR, how can I update it?