Make archive code handle non-zips for custom builds.

[jonathanmetzman]

Or not. But we are getting https://pantheon.corp.google.com/errors/detail/CJ-J2IbVk7_xTQ;time=P30D?project=google.com:clusterfuzz&utm_source=error-reporting-notification&utm_medium=email&utm_content=new-error&e=-13802955&mods=logs_tg_prod now. I think we should probably delete the job but I don't have time to think about this now.