Ensure that scan-policies stage is added after .pre

What does this MR do and why?

This MR fixes a bug when scan-policies stage was not added properly after .pre stage.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Create new project with .gitlab-ci.yml with stages: [.pre] and job using .pre stage.
2. Create new Scan Execution Policy to run on every branch enforcing container-scanning scan.
3. Run pipeline for default branch.
4. See that job for container-scanning is enforced in stage after .pre.

Related to #443593 (closed)