feat: expose is_prebuild_claim attribute

[dannykopping]

Template authors may need to augment their templates to execute scripts once a prebuilt workspace has been claimed.

is_prebuild_claim will be set only when a prebuilt workspace has just been claimed and its subsequent terraform apply is in progress.

Also adds some missing tests to ensure correct input (environment variables) handling.

Can be used in a template like so:

resource "coder_agent" "main" {
  arch           = data.coder_provisioner.me.arch
  os             = "linux"
  startup_script = <<-EOT
    set -e
if [[ "${data.coder_workspace.me.is_prebuild}" = "true" ]]; then
  echo "Created at $(date)" > ~/.prebuild_note # persists across restarts
fi
  EOT

  metadata {
    display_name = "Was Prebuild"
    key          = "prebuild"
    script       = "[[ -e ~/.prebuild_note ]] && echo 'Yes' || echo 'No'"
    interval     = 10
    timeout      = 1
  }
  metadata {
    display_name = "Was Claimed"
    key          = "prebuild_claim"
    script       = "[ '${data.coder_workspace.me.is_prebuild_claim}' = 'true' ] && echo 'Yes' || echo 'No'"
    interval     = 10
    timeout      = 1
  }
}