Add stable_hotfix mode to release recipe

Change-Id: I423bad4e0642da3f722524bbeafbdebb71709ad0
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/447200
Auto-Submit: Ivan Inozemtsev <iinozemtsev@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>