Set policies CI variables with the highest precedence

What does this MR do and why?

This MR changes CI variables precedence for variables defined by security policies and makes them enforced with the highest precedence for scan execution policy jobs, so that they cannot be overwritten. Previously, the policies variables were set as part of each job, so variables on the instance, group, project level or manual pipeline run variables could overwrite them.

This change is done behind feature flag.

Screenshots or screen recordings

Job	Variable value
build-job
dast
sast_iac
dependency-scanning
container-scanning
secrets-detection
sast

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:security_policies_variables_precedence)

2. Go to Secure -> Policies and create a new Scan Execution Policy. Switch to .yaml mode and using the following YAML, define all various scans, each with a different value of the same CI variable:

   type: scan_execution_policy
   name: All scans
   description: ''
   enabled: true
   rules:
     - type: pipeline
       branches:
         - '*'
   actions:
     - scan: dast
       site_profile: ''
       scanner_profile: ''
       variables:
         TEST_TOKEN: my-dast-policy-token
     - scan: sast_iac
       variables:
         TEST_TOKEN: my-sast-iac-policy-token
     - scan: dependency_scanning
       variables:
         TEST_TOKEN: my-dependency-scanning-policy-token
     - scan: container_scanning
       variables:
         TEST_TOKEN: my-container-scanning-policy-token
     - scan: secret_detection
       variables:
         TEST_TOKEN: my-secrets-policy-token
     - scan: sast
       variables:
         TEST_TOKEN: my-sast-policy-token

3. Add a .gitlab-ci.yml file:

   variables:
     TEST_TOKEN: "my-global-token"
     CI_DEBUG_TRACE: "true"
   build-job:
     script:
       - echo "Compiling the code..."
       - echo "Compile complete."
     variables:
       TEST_TOKEN: "my-build-job-token"

4. Define TEST_TOKEN CI variable on various levels:
5. Admin -> Settings -> CI/CD -> Variables with value my-instance-token
6. Group -> Settings -> CI/CD -> Variables with value my-group-token
7. Project -> Settings -> CI/CD -> Variables with value my-project-token
8. Go to Pipelines -> Run pipeline. Add TEST_TOKEN variable with value my-manual-pipeline-run-token
9. Go to the created pipeline and observe the output of each job. Each job should have TEST_TOKEN exported and the last exported value should match the value defined in the policy.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #424028 (closed)