collect checkupdates failures in one pipeline artifact

This is an idea to support https://forum.f-droid.org/t/checkupdatesbot-build-failed-notification-in-f-droid-build-status-app/34536

It collects all checkupdates failures into a single csv file at the end for easier download. A real bonus would be, if this file could be published on https://f-droid.org/repo/status/. I wanted to have a json file at first, but creating a json with a shell script was kind of awkward.