Fix issue with PR build hostname parsing (#8700)
Given a hostname `slug--bug--1.org.readthedocs.build`, the current
parsing code will give a project slug of `slug` and a version slug of
`bug--1`. This was a valid slug creation pattern for a short while,
though we have changed the behavior of the slugging to not produce slugs
with consecutive dashes.
rsplit here searches for the right most instance of double slugs
