Cirrus-CI: build with LLVM 13 package

As of 28a41182c08e the base system uses Clang/LLVM 13. Follow along in
Cirrus-CI (which uses a packaged toolchain for speed).

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 63f44132810d2b15178503cb738866f6e8533aee)