Enable FF_TIMESTAMPS on MR CI jobs

What does this MR do and why?

This MR enables the FF_TIMESTAMPS runner FF (see docs) on MR CI jobs, so that we can dogfood the new functionality before making it more widely available.

Part of #455582 (closed)

Rollout plan issue: #463362 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

See any job from this MR, e.g. https://gitlab.com/gitlab-org/gitlab/-/jobs/7049527037. There is a new column with timestamps on the left side.