Re-consider when to report to build-cop-bot #3821

Currently we send the report in periodic builds.

I think it is better to use more descriptive environment variable, say REPORT_TO_BUILD_COP_BOT or something and we can set that environment variable in our job configurations.

This way, we can have a flexibility of only sending build cop report from particular jobs (although I'm not planning to disable in any of our periodic builds), as well as manually run periodic builds without reporting to build-cop-bot (we can set environment variable when manually running the build).