Fix SecurityPolicyDefault to work with mr pipelines

What does this MR do and why?

Fix SecurityPolicyDefault to work with mr pipelines

References

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• Related to #515224 (closed)

MR acceptance checklist

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Screenshots or screen recordings

How to set up and validate locally

1. Create a new project without a .gitlab-ci.yml file
2. Ensure the Auto DevOps is disabled(Settings > Ci/CD > Auto DevOps)
3. Go to Secure > Policies
4. Click on New policy
5. Select Scan Execution Policy
6. Create a policy to run a secret_detection scan with the latest template:

type: scan_execution_policy
name: policy
description: ''
enabled: true
rules:
  - type: pipeline
    branches:
      - '*'
actions:
  - scan: secret_detection
    template: latest
skip_ci:
  allowed: true
  allowlist:
    users: []

6. Click on Configure with a merge request
7. Merge the new MR to add the policy
8. Create a MR editing the README.md file
9. Verify that a new branch pipeline was created(pipeline without the merge request label) and a MR pipeline was created(pipeline with the merge request label)
10. Add a new commit to the MR
11. Verify that only a MR pipeline was created(pipeline with the merge request label)