Fix container scanning via group level policy

What does this MR do and why?

This fixes a bug where container scanning didn't work when enabled via group level scan execution policy

Related issue: #404513 (closed)

How to set up and validate locally

1. On branch master.

2. Create a new group with a project in it.

3. On the group page, open Security and Compliance -> Policies.

4. Select New policy.

5. Select Scan execution policy.

6. Switch to yaml. mode.

7. Paste the security policy:

   type: scan_execution_policy
   name: test
   description: ''
   enabled: true
   rules:
     - type: schedule
       agents:
         test-agent:
           namespaces:
             - test
       cadence: 0 0 * * *
   actions:
     - scan: container_scanning
       tags: []

8. Select Configure with a merge request.

9. Merge the MR.

10. Open a rails console.

11. Create a new KAS JWT using the rails console:

    JWT.encode({ 'iss' => Gitlab::Kas::JWT_ISSUER }, Gitlab::Kas.secret, 'HS256')

12. Find the project belonging to the group project = Project.find(PROJECT_ID)

13. Create new agent:

    agent = Clusters::Agent.new(project_id: project.id, created_by_user: User.find(1), name: "test-agent")
    agent.save!

14. Create new agent token and retrieve the value

    token = Clusters::AgentToken.new(agent: agent, created_by_user: User.find(1), name: "test-agent-token")
    token.save!
    token.token

15. Send the API request (replace $KAS_JWT and $AGENT_TOKEN):

curl --include \
     --header "Gitlab-Kas-Api-Request: $KAS_JWT" \
     --header "Authorization: Bearer $AGENT_TOKEN" --header "Content-Type: application/json" \
     --url "http://gdk.test:3000/api/v4/internal/kubernetes/modules/starboard_vulnerability/policies_configuration"

1. This should result in an empty response {"configurations":[]}
2. Switch to the branch 04513-fix-group-level-container-scanning
3. Execute the curl statement again.
4. This should now return the security policy like {"configurations":[{"cadence":"0 0 * * *","namespaces":["test"],"updated_at":"2023-05-08T09:19:52+00:00"}]}

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 #404513 (closed)