Problem using lookup in a source block

Packer
1

I’ve been trying to figure out how to help create a kind of opinionated template using packer. Essentially I want to use a base source as a “template” and make it work with some pre-provided data but I want some of the values to be able to be overridden for the builds that are prescribed, i.e. iterate through a local defined object but allow a defined variable to work using the lookup function

locals {
  build_types = {
    test_build1 = {
      test1    = "value1"
      ssh_user = "myuser1"
    }
    test_build2 = {
      test1    = "foo"
      ssh_user = "my_user2"
    }
  }
  test_lookup = {
    null = "myval"
    test = "mytest"
  }
}

variable "testvar" {
  type    = string
  #default = ""
}

source "null" "template" {
  ssh_host = "127.0.0.1"
  ssh_password = "password"
}

build {
  name = "testing"
  description = "dynamic build testing"

  dynamic "source" {
    for_each = local.build_types

    labels   = ["source.null.template"]
    content {
      name   = source.key

      ssh_username = lookup(var, testvar, source.value.ssh_user)
    }
  }

  provisioner "shell-local" {
    inline = [
      "echo Build Name is ${source.name}",
      "echo Test1 Value is ${local.build_types[source.name].test1}",
      "echo Lookup Value is ${lookup(local.test_lookup, source.type, local.test_lookup["test"])}"
    ]
  }
}

I get different errors on the commented section. If I keep it as is (with the comment), it complains that the variable is unset:

❯ packer validate .
Error: Unset variable "testvar"

A used variable must be set or have a default value; see
https://packer.io/docs/templates/hcl_templates/syntax for details.

If I uncomment the line, it gives a different error

❯ packer validate .
Error: Unknown variable

  on file.pkr.hcl line 39:
  (source code not available)

There is no variable named "testvar".

Error: Unknown variable

  on file.pkr.hcl line 39:
  (source code not available)

There is no variable named "testvar".

From what I can tell, it’s because you are not allowed to use the lookup function in a source block at all. Has anyone else found a way to work around this type of scenario, or am I trying to do something that shouldn’t be possible?