CI: Restructure PR HTML generatorThis splits the workflow into two. The main functionality testing the PRis run as unprivileged pull_request workflow and uploads the results asartifacts. The privileged workflow_run workflow takes these, uploadsthem to GH Pages and writes/updates a comment. The latter does notinteract with the PR content any further.Controversial change: Try to use the prebuilt base/current containerinstead of building it again inside of this action. With unfavorabletiming, this can lead to spurious errors.This fixes https://github.com/RSS-Bridge/rss-bridge/issues/5048