Make pipeline execution policy variables take precedence

What does this MR do and why?

Variables defined in a pipeline execution policy will take the highest precedence. This MR makes it work for variables that are defined as part of a custom scan action

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 Run customized CI YAML file as security policy actions
5. Create a project
6. Add a .gitlab-ci.yml file containing a variable:

   variables:
       GLOBAL_VAR: "project setting"

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: |-
          variables:
            GLOBAL_VAR: "policy"
          test policy job:
            stage: test
            script:
              - echo "'$GLOBAL_VAR'"

11. Select Configure with a merge request and merge the MR
12. On the project page, Select Build -> Pipelines
13. Select Run pipeline
14. Add a variable with key: GLOBAL_VAR and value: web trigger
15. Inspect the test policy job logs. The printed variable should say "policy" and not "web trigger".

Related to #439237 (closed)