Update sizebot to match name of CircleCI build job (#15714)

The sizebot scrapes the GitHub `/statuses` endpoint to get the lastest
CircleCI build number for master, in order to fetch the bundle size
info for that build, which are stored as build artifacts. (There's gotta
be a better way to do this, but that's what we have for now.) This
updates the script to match the name of the updated CircleCI job that
generates the bundle sizes.