build_article() doesn't set IN_PKGDOWN

[jflournoy]

Perhaps this is the intended behavior, but if not, I want to point out that build_articles calls section_init, which ends up setting IN_PKGDOWN to true, but build_article calls as_pkgdown, which does not end up setting IN_PKGDOWN at all. I think it would be useful to be able to check if IN_PKGDOWN is true when building single articles (well, at least for me).