Pipeline Execution Action with external CI fails with "Undefined error"

Summary

Creating pipelines for projects with a pipeline Execution policy fails with "Undefined error" if the policy uses an external CI file.

Steps to reproduce

1. Create a new Group.
2. Navigate to Settings -> General.
3. Expand the Permissions and group features section.
4. Enable the "Run customized CI YAML file as security policy actions" experiment.
5. Create a new project in the group.
6. Add a new file called 'ci.yml' to the project with content:

   Security Policy CI Job:
     stage: test
     script:
       - echo "This defined in a scan execution policy"

7. Create another project in the group with a simple ci file:

   Job From Project CI:
     stage: test
     script:
       - echo "Ci job"

8. Select Secure -> Policies -> New policy .
9. Select Scan execution policy.
10. Choose a name.
11. Select Add new action.
12. Select Run CI/CD code.
13. Select Run linked existing CI file.
14. Select the ci.yml file from the first project you created.
15. Select Configure with a merge request and merge the MR.
16. Go back to the previous project and start a new pipeline.
17. This should fail with "Undefined error".

Example Project

https://gitlab.com/gitlab-org/govern/security-policies/andys-test-group/custom-ci-yaml-demo/test

Possible fixes

When fetching CI variables, we only take the ci_configuration option into account (ee/app/services/security/security_orchestration_policies/scan_pipeline_service.rb:55). With an external CI file, the action uses the ci_configuration_path option instead. In this case, we need to load the external file and fetch variables from there.