flaky infrastructure should work for all branches

We need a flaky list per-branch, since what does/does not work reliably on master isn't really relevant to our older maintenance release lines.

We should just make the invocation a step in the current per-branch nightly jobs, prior to when we need the list in the stages that run unit tests. We can publish it in the nightly job as well so that precommit can still get it. (and can fetch it per-branch if needed)