cirrus: Force pkg update on FreeBSD.

Seems like FreeBSD ports/images are not well maintained and frequently
causes package installation failures like this:

 [1/40] Fetching automake-1.16.1_2.txz: .......... done
 pkg: cached package automake-1.16.1_2: size mismatch, fetching from remote
 [2/40] Fetching automake-1.16.1_2.txz: .......... done
 pkg: cached package automake-1.16.1_2: size mismatch, cannot continue
 Consider running 'pkg update -f'

Forced update doesn't increase build time significantly, but helps
to solve at least this one kind of issues.

Acked-by: William Tu <u9012063@gmail.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

.cirrus.yml

@@ -17,6 +17,7 @@ freebsd_build_task:
 
   prepare_script:
     - sysctl -w kern.coredump=0
+    - pkg update -f
     - pkg install -y ${DEPENDENCIES}
 
   configure_script: