Cirrus CI: cache more of ~/.cargo
Cirrus CI: cache more of ~/.cargo

CI builds sometimes fail to download the registry from GitHub, which is
annoying. This commit extends the cache to include the registry. If
I understand Cargo's layout correctly, CI won't touch GitHub anymore
unless there is a new dependency that hasn't been fetched yet.

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