Set HUGO_BASEURL for building www-staging-k-o
CSS and fonts are not loaded on www-staging.kde.org due to cross-origin requests to resources on kde.org. This in turn is because the base URL is the same on both master and staging branches.
Setting HUGO_BASEURL has the same effect as changing the config field, while we don't need to make changes to the website repo and people can keep their usual workflow of synchronizing those two branches.