Cache bucket ARN not found when cache_enabled = "false"

[roryhow]

It seems that when cache is disabled (i.e cache_enabled = "false"), terraform plan will fail with the following output:

Error: Error running plan: 1 error(s) occurred:

* module.build.data.aws_iam_policy_document.permissions_cache_bucket: 1 error(s) occurred:

* module.build.data.aws_iam_policy_document.permissions_cache_bucket: Resource 'aws_s3_bucket.cache_bucket' not found for variable 'aws_s3_bucket.cache_bucket.arn'

my assumption is that this is due to the following aws_iam_policy_document missing a check for var.cache_enabled. I made a fork where the permissions_cache_bucket is updated like so:

data "aws_iam_policy_document" "permissions_cache_bucket" {
  count = "${var.enabled == "true" && var.cache_enabled == "true" ? 1 : 0}"

  statement {
    sid = ""

    actions = [
      "s3:*",
    ]

    effect = "Allow"

    resources = [
      "${aws_s3_bucket.cache_bucket.arn}",
      "${aws_s3_bucket.cache_bucket.arn}/*",
    ]
  }
}

Please let me know if I'm doing something wrong (I'm pretty new to Terraform so this could definitely be the case), or if it's a bug, I can submit a PR. Thanks!