Change `set_enforcement_limit` to work with `build` groups

What does this MR do and why?

Groups created with build in tests don't have the gitlab_subscription object/record, so we can't use set_enforcement_limit

  1) Namespace#actual_size_limit when in namespace enforcement returns the plan limit
     Failure/Error: set_enforcement_limit(namespace, megabytes: enforceable_storage_limit)

     NoMethodError:
       undefined method `hosted_plan' for nil:NilClass

           namespace.gitlab_subscription.hosted_plan.actual_limits.update!(enforcement_limit: megabytes)

Screenshots or screen recordings

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How to set up and validate locally

• Pipelines being green are enough validation

MR acceptance checklist

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