rustdoc does not forward lint settings to the doctest builds

[RalfJung]

TIL that while a bunch of flags are forwarded to doctests builds (including --cfg and --check-cfg), this is not true for --cap-lints as well as -A/-D and the other lint flags. This caught me by surprise when I tried to suppress a deny(warnings) in a doctest. See Zulip for the surrounding discussion.

Would it make sense to also forward the lint flags to the test build? That would seem to make things more consistent.