Cirrus-CI: add gcc12 automatic task on primary GitHub mirror

We want to get GCC coverage via Cirrus-CI, but don't want to trigger
excessive runs across all forks and branches. Create a duplicate gcc12
task to run automatically for freebsd/freebsd-src.

Reviewed by: jhb
Sponsored by: The FreeBSD Foundation

(cherry picked from commit f5f1b9a9828a4fdf5b0ba51b291e0e1e7ac7b55f)