Cirrus-CI: use FreeBSD 12.2 image

Direct commit to stable/12 as main has other changes here, using the
llvm package to avoid building the toolchain.

(cherry picked from commit 046f91f21ac619f7307a8e853f863ded4df7f322)

Note that this does not have any effect on built artifacts (and thus is
not an EN candidate) but allows Cirrus-CI to be used on this releng
branch.

Approved by: so