Fix error when rendering project pipeline status

What does this MR do?

project.last_pipeline always queries the DB for the last pipeline so the project.last_pipeline.present? check does not ensure that project.last_pipeline exists when we call project.last_pipeline.detailed_status(current_user).

We fix this by saving the last pipeline to a local variable so that #present? and #detailed_status are called on the same object.

Related to #321939 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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