Fix policy template to not modify strings in place

What does this MR do and why?

This is a follow-up of !2572 (merged) where I applied suggestions without checking that generated policies would still generate with the expected output.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review