Stub env with string value

• Ensure stub_env will return the stubbed value as string.
• Also fix bug in https://gitlab.com/gitlab-org/quality/triage-serverless/-/issues/166#note_684949305

Action items

• When adding a new Triager, verify that it has been added to the DEFAULT_TRIAGERS in triage/triage/handler.rb in the appropriate order.
• If applicable, identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Week-in-review