Avoid deep merging security policy CI

What does this MR do and why?

CI configured in a pipeline execution policy ('custom' scan type), will be merged with the project CI before a pipeline runs. A deep merge, means that attributes can be injected in a job from the project CI.

For example, if the project CI defines a job with the same title as a security policy CI with a 'before_script'. The before script will be executed as part of the job.

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 a group
2. Go to Settings -> General
3. Toggle the Permissions and group features section
4. Enable Security policy pipeline execution action
5. Create a project
6. Add a .gitlab-ci.yml file containing a variable:

   build job:
     stage: build
     script:
       - echo "Project CI"
     before_script:
       - echo "Injected from project CI"

7. Go to Secure -> Policies
8. Select New policy
9. Select Scan execution policy
10. Switch to .yaml mode and paste the following policy:

    type: scan_execution_policy
    name: Compliance
    description: ''
    enabled: true
    rules:
      - type: pipeline
        branch_type: all
    actions:
      - scan: custom
        ci_configuration: |-
          build job:
            stage: build
            script:
              - echo "Security policy CI"

11. Select Configure with a merge request and merge the MR
12. On the project page, Select Build -> Pipelines
13. Select Run pipeline
14. Inspect the build job logs.
15. Observe that the before script defined in the project CI is not executed.

Related to #439337 (closed)