Trial builds cant handle rate limiting on build requests when doing more than one build at a time

https://github.com/google/oss-fuzz/commit/871ea5240a4cadbc9fd9a78cf5b1e258734fe6cf was an easy fix but it doesn't work at all when there are multiple branches being tested concurrently.
We need to add a retry.

Start by reviewing commit 871ea5240a4cadbc9fd9a78cf5b1e258734fe6cf and tracing the trial-build request path that it changed. Reproduce concurrent builds with rate limiting enabled, then verify that requests retry successfully when multiple branches are tested concurrently.

Written by the indexing model from the issue text.