fix: Set EKS cluster name in Terraform self-service instructions

[progerjkd]

export EKS_CLUSTER_NAME

What this PR does / why we need it:

Add the required step to define EKS_CLUSTER_NAME.

Which issue(s) this PR fixes:

Missing variable definition for EKS_CLUSTER_NAME.
Fixes #

Quality checks

• My content adheres to the style guidelines
• I ran make test module="<module>" it was successful (see https://github.com/aws-samples/eks-workshop-v2/blob/main/docs/automated_tests.md)

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[progerjkd]

make test module="autoscaling"

[bryanmorganoverbey]

I may open another PR for this, but while we are at it, it would help to go ahead and set the kube config file properly after adding the terraform resources:
aws eks --region us-east-2 update-kubeconfig --name eks-workshop

This isn't mentioned in the docs, but it is a necessary step