Cirrus-CI: temporarily disable amd64-gcc12 build/test on main

The amd64-gcc12 job is failing at present. Unfortunately Cirrus-CI
appears to have a 100MB log limit and we exceed this before the error
is reported. Disable the job for now until we can address the log
length issue.

See also https://github.com/cirruslabs/cirrus-ci-docs/issues/1176.

Sponsored by: The FreeBSD Foundation