Cirrus CI: lower concurrency to 10 jobs
Cirrus CI: lower concurrency to 10 jobs

12 jobs is a bit too much for 4GB of RAM, here's a task being killed by
OOM killer: https://cirrus-ci.com/task/6714709614264320

der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat