Fortunately, currently with the gitiles poller scheduling post-submit builds, things "Just work".

Problem

1. Release builders are clones of master builders
2. {try,prod}_builders.json builder names are forked
3. sharded tests have been flattened at the top level, with the total number of shards in their name
4. thus, once we start scheduling from prod_builders.json/.ci.yaml, release shards that have a different number of sub-shards than master branch will all fail to identify a builder

Solutions

1. Refactor builder config to have forked config for each branch
2. re-consolidate sub-sharded targets into one entry in .ci.yaml with a property for number of sub-shards, and let cocoon manage scheduling multiple builds
3. cherrypick upstream .ci.yaml targets config to release branch