cirrus.yml: Update the FreeBSD task to version 12.2

FreeBSD version 12.1 is out of service now, and the task in the
Cirrus-CI is failing. Update to 12.2 to get it working again.
Unfortunately, there is a bug in libtasn1 that triggers with the
new version of Clang that is used there (see this thread for details:
https://lists.gnu.org/archive/html/qemu-devel/2021-02/msg00739.html

 ),
so we have to disable gnutls for now to make it work again. We can
enable it later again once libtasn1 has been fixed in FreeBSD.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210319101402.48871-1-thuth@redhat.com>
Message-Id: <20210323165308.15244-13-alex.bennee@linaro.org>