ci: enable flakybot for test failure detection

We should install the [flakybot](https://github.com/googleapis/repo-automation-bots/tree/main/packages/flakybot) app to notify us of failing tests in our continuous job.

@vishwarajanand has identified a set of already failing tests. Once we do a bit of clean up, we can start the process of enabling the app on the repo. Those tests that aren't addressed initially should result in new issues being created.