Fix security_policy_bot CI job access for private projects

What does this MR do and why?

This MR fixes an issue where scheduled pipelines from scan execution policies in private projects would have failing job due to unauthorized access.

How to set up and validate locally

1. Enable the scan_execution_bot_users feature flag.
2. Prepare the test project. Follow the steps below:
   1. Create a new private project with a working .gitlab-ci.yml file. For example, use the bash template.
   2. Add an empty named package.json to the project. This is required for the SAST scan to run.
   3. On the left sidebar select Repository and Branches
   4. Select New branch
   5. Enter test as name
   6. Select Create branch
3. Prepare the security policy
   1. On the left sidebar, select Security and Compliance* and Policies.
   2. Select New policy.
   3. Select Scan execution policy
   4. Switch to .yaml mode
   5. Replace the content with the example policy yaml below
   6. Select Configure with a merge request
   7. Select Merge.
   8. Go back to your initial project
   9. There should now be a bot member named GitLab Security Policy Bot.
4. Wait for the next full hour until the scheduled pipeline is triggered, or trigger the job manually:

   Security::OrchestrationPolicyRuleSchedule.last.update_columns next_run_at: 1.day.ago
   Security::OrchestrationPolicyRuleScheduleWorker.new.perform

5. View the pipelines page of your project
6. There should be a new pipeline triggered by the GitLab Security Policy Bot
7. The pipeline should pass

Example policy yaml

type: scan_execution_policy
name: test
description: ''
enabled: true
rules:
  - type: schedule
    branches:
      - test
    cadence: 0 * * * *
actions:
  - scan: sast
    tags: []

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 #414278 (closed)