Cirrus CI: switch to MacOS 10.15 (Catalina)
Cirrus CI: switch to MacOS 10.15 (Catalina)

Cirrus CI replaced their 10.14 (Mojave) images with 10.15; this commit
simply updates the config to avoid auto-upgrade notifications during
builds.

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