[dartdoc] Upload latest also for the current stable version

When the current stable is re-released, it's likely to fix some issue with the release. So it makes sense to also re-publish the dartdocs to latest (which is what will be served from api.dart.dev's root).

Bug: b/320447980
Change-Id: Idc280402dae3285200632f0ea52b16992d6b478e
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/368600
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>