Upload new custom builds to a specified bucket when available.

[jonathanmetzman]

This will allow the untrusted worker to access the builds without special handling. Fixes: https://github.com/google/clusterfuzz/issues/3957

[jonathanmetzman]

I suppose we could force this for all users of ClusterFuzz. Note this approach will break old builds of past custom builds. But we can reupload them to get them working (regression task might not work though, I probably need to copy them to the new bucket)