Enforce SEP jobs when `pipeline_source` is a string

What does this MR do and why?

Enforce SEP jobs when pipeline_source is a string.

This fixes a case when SEP jobs are not enforced when pipeline is run manually via UI and the project doesn't have a CI configuration.

It was likely caused by !174577 (merged) which changed the pipeline source when triggering the pipeline from :web to 'web'.

References

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MR acceptance checklist

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How to set up and validate locally

1. Create a project
2. Create a SEP with secret_detection to run on all branches
3. Go to the project /pipelines/new and run a pipeline
4. Verify that pipeline runs with secret-detection-0 job

Related to #517978 (closed)