erl not available in Heroku CI debug shell

To reproduce

• git clone https://github.com/HashNuke/heroku-buildpack-elixir-test.git
• cd heroku-buildpack-elixir-test
• Add the following app.json file:

{
  "name": "buildpack-test",
  "scripts": {},
  "env": {},
  "formation": {},
  "addons": [
    {
      "plan": "heroku-postgresql"
    }
  ],
  "buildpacks": [
    { "url": "https://github.com/HashNuke/heroku-buildpack-elixir.git" }
  ]
}

• git add app.json
• git commit -m "Add app.json"
• heroku create --buildpack "https://github.com/HashNuke/heroku-buildpack-elixir.git"
• git push heroku master (not sure if needed)
• Create a Github repo and associate it to the CI of your new Heroku app.
• git push origin master should trigger the test runner.
• heroku ci:debug --pipeline [pipeline-name]
• In the remote prompt: erl

Current behavior

• bash: erl: command not found

Expected behavior

• Erlang shell should start

Reproduce the issue using the listed Heroku CI commands and the app.json configuration, then enter the debug shell and run erl. Trace the buildpack's installed runtime commands and verify that the Erlang shell is available in the debug environment; done means erl starts successfully there.