Eagerly queue up failed builds from old rustdoc versions when rustdoc is updated

[syphar]

coming from #301

Eagerly queue up failed builds from old rustdoc versions when rustdoc is updated, at a lower priority than user requests. For the same reason as before, this lets us try to eagerly rebuild crates that may have failed due to the old rustdoc, while not clogging up the queue for new releases.

now added by me:
I have the feeling (that needs data) that most of the build failures aren't related to rustdoc issues that are fixed with newer versions. This means the impact of this automation might be wasted build time.