change_mode="script" not working with techpreview2

[flz]

Here's the test repro:

$ cat foo.nomad.tpl
job "foo" {
  datacenters = ["xxx"]
  group "foo" {
    task "foo" {
      driver = "docker"
      config {
        image = "alpine"
      }

      template {
        data = "foo"
        destination = "local/foo.txt"
        change_mode = "script"
        change_script {
          command = "/bin/true"
        }
      }
    }
  }
}

As shown below, nomad-pack plan fails but nomad-pack render + nomad plan seems to work just fine.

$ nomad-pack plan ../../packs/foo
! Failed To Perform Plan

        Error:   1 error occurred:
        * Task group foo validation failed: 1 error occurred:
        * Task foo validation failed: 1 error occurred:
        * Template 1 validation failed: 1 error occurred:
        * must specify change script configuration value when change mode is script

        Type:    *errors.errorString
        Context: 
                 - Registry Name: dev
                 - Pack Name: foo
                 - Pack Ref: dev
                 - Deployment Name: foo
                 - Template Name: foo/templates/foo.nomad.tpl
                 - Job Name: foo

$ nomad-pack render ../../packs/foo -o .
foo/foo.nomad:

job "foo" {
  datacenters = ["xxx"]
  group "foo" {
    task "foo" {
      driver = "docker"
      config {
        image = "alpine"
      }

      template {
        data = "foo"
        destination = "local/foo.txt"
        change_mode = "script"
        change_script {
          command = "/bin/true"
        }
      }
    }
  }
}

$ nomad plan ./foo/foo.nomad
+ Job: "foo"
+ Task Group: "foo" (1 create)
  + Task: "foo" (forces create)

Scheduler dry-run:
- All tasks successfully allocated.

Job Modify Index: 0
To submit the job with version verification run:

nomad job run -check-index 0 ./foo/foo.nomad

When running the job with the check-index flag, the job will only be run if the
job modify index given matches the server-side version. If the index has
changed, another user has modified the job and the plan's results are
potentially invalid.