Trigger policy workers based on diff_head_pipeline

What does this MR do and why?

We need to check diff_head_pipeline instead of head_pipeline_id when we trigger policy sync worker so that the merge check is correctly updated.

If there's a second commit in the MR which doesn't trigger pipelines, head_pipeline points to the pipeline for the first commit and diff_head_pipeline is nil.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Create a project with the following .gitlab-ci.yml

           workflow:
             rules:
               - if: $GITLAB_USER_LOGIN == "<username_to_impersonate>"
                 when: never
               - if: $CI_PIPELINE_SOURCE == "merge_request_event"
               - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
               - if: $CI_COMMIT_BRANCH && $CI_OPEN_MERGE_REQUESTS
                 when: never
           
           variables:
             AST_ENABLE_MR_PIPELINES: "true"
           
           include:
             - template: Jobs/Secret-Detection.gitlab-ci.yml
           
           test:
             script:
               - echo ""
           

2. Set up a merge request approval policy

   approval_policy:
     - name: Test
       description: ''
       enabled: true
       actions:
         - type: require_approval
           approvals_required: 1
           role_approvers:
             - maintainer
             - owner
         - type: send_bot_message
           enabled: true
       rules:
         - type: scan_finding
           scanners: []
           vulnerabilities_allowed: 0
           severity_levels: []
           vulnerability_states: []
           branch_type: protected
       fallback_behavior:
         fail: closed    

3. Create a new branch and push a commit.

4. Open a merge request.

5. Impersonate the user for whom the pipelines are disabled

6. In the merge request, push another commit

7. Make sure that the policy merge check fails and doesn't get stuck

MR acceptance checklist

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

Related to #589614