Cirrus-CI: add gpart show to setup script

GCP instances have stopped resizing the UFS root filesystem. The 14.x
snapshot image built from b1ef176eb528 works while c89209c674f2 fails.
Discovered via testing a WIP branch in Cirrus-CI that uses
image: family/freebsd-14-0-snap.

Add gpart show to the setup script to confirm that the partition
size (as set by Cirrus config / GCP) took effect and to help when
investigating any similar issue in the future.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation