Attempt to fix @gitlab/ui changelog fetching

It seems that since the move to a monorepo, Renovate isn't parsing out the sourceUrl for the @gitlab/ui package. For now, work around that by explicitly setting it. From the docs:

Source URLs are necessary to link to the source of the package and in order to look up changelogs.

See gitlab-org/gitlab!194930 (comment 2573912621) for more context.