Cirrus-CI: avoid boot smoke test on stable/12

The boot smoke test boots from a FAT filesystem (created in-memory
by QEMU), but this does not work on stable/12:

mountroot: waiting for device /dev/ada0s1...
Mounting from msdosfs:/dev/ada0s1 failed with error 19.

For now on stable/12 use Cirrus-CI as only a build test.

Sponsored by: The FreeBSD Foundation