This dogfood artifacts and download the Knapsack report, flaky specs
report, and Crystalball report from the latest successful master
pipeline triggered by @gitlab-bot (this should be a scheduled pipeline
for which we're sure update-tests-metadata was run).

This also remove the need to upload these reports to S3 since they're
already uploaded as artifacts.
Signed-off-by: Rémy Coutable <remy@rymai.me>