Using input variables for runner data_source config

Hello!

Is it possible to use input variables for the runner configs data_source? When I run waypoint init -update with the below waypoint.hcl file, i get the error There is no variable named "var".

variable "github_token" {
  type = string
}

runner {
  enabled = true

  data_source "git" {
    url              = "https://github.com/example/example.git"
    username = "git"
    password = var.github_token
  }
}

Am I doing something wrong or is it simply not supported? If so, why not?