fix(ec2): onePerAz does not work for looked-up VPCs

[rix0rrr]

Fix the use of onePerAz for VPCs obtained through Vpc.fromLookup().

Fixes #3126.

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[hardcode83]

Hi guys, @rix0rrr

I am using ecsPatterns.NetworkLoadBalancedFargateService with a vpc imported with vpc lookup and I am having the same problem yet.

PS C:\pod\repos\cdk-ecs-networkloadbalancer> cdk --version
2.38.1 (build a5ced21)

export class EcsNetworkFargate extends Construct {
constructor(scope: Construct, id: string, props: EcsNetworkFargateProps) {
super(scope, id);
const repo = Repository.fromRepositoryArn(this, "repo", props.arnRepo)
const vpc = ec2.Vpc.fromLookup(this, "VPC", {
vpcId: props.vpcId
})
const hostedZone = route53.HostedZone.fromLookup(this, 'hosted_zone', {
domainName: props.domainZone
});

const loadBalancedFargateService = new ecsPatterns.NetworkLoadBalancedFargateService(this, 'gd-billing', {
  vpc: vpc,

  assignPublicIp: props.assignPublicIp,
  taskSubnets:
  {subnetType: ec2.SubnetType.PUBLIC,
    onePerAz: true,
  
  }

}
}

**I am tryng filter with the several options of taskSubnets but always get the same error:

AWS::ElasticLoadBalancingV2::LoadBalancer | staging/sdf/LB (asdfLB6A0E60DF) A load balancer cannot be attached to multiple subnets in the same Availability Zone (Service: AmazonElasticLoadBalancing; Status Code: 400; Error Code: InvalidConfigurationRequest; Request ID: 303a030b-0ec7-4dbd-afe1-195b3089c529; Proxy: null)

Thx you su much in advance!