test-infra: `/cherrypick ` doesn't work in k/k

Can we enable the cherrypick plugin for the CI infrastructure that allows /cherrypick <branch> to cause the bot to cherrypick the pull on top of the chosen branch, create a new pull, and assign it. It’s a really handy one to have for maintenance.

@kargakis @stevekuznetsov what’s that plugin called?

/cc @dims