How can I create an EKS fargate cluster using pulumi? I know how to create an EKS cluster:

# Create an EKS cluster with the default configuration.
cluster = eks.Cluster(
    "theia-prod",
    eks.ClusterArgs(),
)

However, how do I specify for it to use fargate?

did you check the docs? https://www.pulumi.com/registry/packages/eks/api-docs/cluster/#fargate_nodejs