eks-prow-build-cluster: Use Karpenter instead of cluster-autoscaler

[xmudrii]

cluster-autoscaler doesn't support mixing different instance types. This is a problem because r5ad.4xlarge is way too expensive ($1.048/hr) and we might want to combine bigger and smaller instances (e.g. not all tests need such large instances).

We also might want to consider mixing on-demand and spot instances for additional savings, which Karpenter would allow us to do.

/priority important-soon

[xmudrii]

/priority important-soon

[xmudrii]

/milestone v1.28
/sig k8s-infra
/area infra
/area infra/aws
/kind cleanup

[sbdtu5498]

/assign

[xmudrii]

@sbdtu5498 Are you still interested to work on this issue?