Previews aren't building due to HTTP 401 error

Continuing the discussion from eclipsefdn/it/webdev/solstice-assets!575 (comment 2935317)

A request to the GitLab API occurs in the "Compute build variables" step, resulting in an HTTP 401. This is failing the build and preventing all of our Hugo sites from generating previews.

@heurtemattes mentioned it could be because I was using a fork. From what I understand on https://gitlab.eclipse.org/eclipsefdn/infrazilla/-/issues/1332, we tightened permissions around triggering previews for merge requests from forks. However, this 401 error still occurs for merge requests from branches on the origin repo. So, there is undoubtedly something going wrong here.

cc @cguindon