Cirrus-CI: use llvm15 toolchain packages

As of commit 50d7464c3fe6 we use llvm15 as the system toolchain, and
commit eca005d8531f added compiler options incompatible with earlier
versions. Switch to llvm15 packages.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 67fca20eb2cea55fb2eaacc22807ee4741894796)