Cirrus-CI: switch to llvm18 by default

As of commit 439352ac8257 Clang/LLVM 18 is the default in-tree compiler.
Follow suit in with the external toolchain package used by Cirrus-CI.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit e8b7c78c1b38d0486ff12993c0529a201030bd07)