step field no longer work with terraform 0.12.x

[runlevel5]

Context

Given:

resource "buildkite_pipeline" "terraform_test" {
  name       = "terraform-test"
  repository = "[email protected]:you/repo.git"

  step = [
    {
      type    = "script"
      name    = ":llama: Tests"
      command = "echo Hello world!"
    },
  ]
}

When applying with terraform 0.12, it would yield following error:

Error: Unsupported argument

  on demo.tf line 5, in resource "buildkite_pipeline" "terraform_test":
   5:   step = {

An argument named "step" is not expected here. Did you mean to define a block
of type "step"?

It might be related to the breaking syntax changes. Ref: https://www.terraform.io/docs/extend/terraform-0.12-compatibility.html#configuration-syntax-changes