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

[emaste]

Summary

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

[emaste]

freebsd-12-1-snap: https://cirrus-ci.com/task/5348410082459648
freebsd-13-0-release-amd64: https://cirrus-ci.com/task/4802977186709504

The two runs completed in about the same amount of time (33:17 for 12.1, 31:29 for 13.0).

[lwhsu]

Note with this change, we're switching from using latest -STABLE to -RELEASE.

[emaste]

Note with this change, we're switching from using latest -STABLE to -RELEASE.

Yes - we were using -STABLE primarily because the GCP 12.2 release image was broken.

[emaste]

Cirrus-CI main branch build times https://cirrus-ci.com/github/freebsd/freebsd-src/main

Rightmost 4 green bars are after this change.
(Failures on this change and the successive one were transient git clone failures)