Avoid duplicate pipeline due to container scanning job

What does this MR do?

Avoid duplicate pipeline due to missing rule in container_scanning job. This fixes a regression of !61850 (merged).

You can see the problem in gitlabracadabra/gitlabracadabra!172 (merged) for example, were the MR was merged with a broken pipeline, because another pipeline with only this container_scanning was successful (more precisely allowed to fail).

For more context, see !61850 (comment 587800712). Customers are affected too.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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