Instructions to find CI artifacts in "Testing pull requests" are outdated

[akien-mga]

Issue description:
The instructions we have to download CI artifacts in the "Testing pull requests" page are outdated, as the GitHub Actions interface changed a while ago.

We need to update the screenshots and text to make sure users can find artifacts, as it's far from obvious, even more so in the current interface.

URL to the documentation page:
https://docs.godotengine.org/en/latest/contributing/workflow/testing_pull_requests.html

[Calinou]

We could recommend using nightly.link for all users (even those with a GitHub account), as its UX is likely better (and won't be affected by GitHub Actions UI changes).

[tetrapod00]

Seemingly fixed in #7702.