Investigate why `review-build-cng` is taking 30 minutes when it was taking 20 minutes until June

As can be seen in https://app.periscopedata.com/app/gitlab/496118/Engineering-Productivity-Sandbox?widget=6914314&udv=785399, the review-build-cng job was taking around 20 minutes until approximately June 14th, and then it went up to approximately 30 minutes:

We should try to understand what's causing this and improve it to go back to at least 20 minutes, and ideally less.