Prevent CreatePipelineService from running inside a transaction

Following up from a conversation: !16276 (diffs, comment 216879915)

Problem

We should not run CreatePipelineService inside an existing transaction for reasons described here: gitlab-foss#65281 (comment 213536236)

Proposal

Raise an error immediately inside CreatePipelineService#execute to prevent it from running. This is aimed to prevent developers from using the service wrongly as well making constraints explicit.