build_manifest: Add the ability to pick a slightly older cerbero commit (one with build deps ready)

Right now we have a slight race in the CI flow with cerbero. If someone push to cerbero, and right next a MR is create on another repo, the new MR CI won't find a build cache for the cerbero commit picked by build_manifest.py. That build is not ready yet and takes a bit of time to produce. This will then create more load on the CI and will slow down everyone (including the build producing the deps).

In order to avoid the unneeded delay, I'd like to introduce a bit of cerbero specific code into the manifest creation. Basically, similarly to how cerbero finds the caches, I would like to download the build log, which is a simple json file, and if it's found, I would pick the latest commit from that log (the top one). I'm writing it down now since I wanted to do that for a while and I kind of forgot about it couple of times already.

The JSON log can be found here. It doe not matter which one you pick, since the public links for "latest" artifacts only gives you access to the artifacts when the entire pipeline have succeeded. Which is exactly what we are looking for.