Fixed source path rendering for 'pure' yml policies

What does this MR do and why?

In Don't assume group label prefix (!2614 - merged) a change was introduced that renders the source policy path dynamically. This is valid for some of the files, which are dynamically generated through an ERB template, however, some policies which are 'plain' yaml fail to render.

This fixes the 'plain' yaml policies to use the variable TRIAGE_POLICY_FILE which should be already available to them.

Fixes #1471 (closed)

Expected impact & dry-runs

Affected policies should render their source path properly.

Dry run confirms this: https://gitlab.com/gitlab-org/quality/triage-ops/-/jobs/6051632417

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