Try to update ci_builds.artifacts_size for old builds

What does this MR do?

We introduced ci_builds.artifacts_size in !4964 (merged) (#18869 (closed)), so we should fill it for builds before this taking effect.

Are there points in the code the reviewer needs to double check?

I don't know how to test this! I think neither can we test this on staging because we don't have artifacts on staging. We could see if this would keep complaining File is unexpectedly missing for... on staging though!