Cirrus-CI: use FreeBSD 13.0 image for base system CI build

We generally want to build and test on the highest release version, and
FreeBSD 13.0 also brings some performance benefits.

Reviewed by: lwhsu
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29842

(cherry picked from commit 407abff2b91847e23711625ad7c69c17e99b3d1d)