Fix flaky artifacts test

What does this MR do and why?

This test is flaky because artifact.file.url contains a timestamp based on Time.now, which can change between when the URL is generated in the response and the assertion.

This MR fixes the flakiness (hopefully) by using the freeze_time helper.

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.