Bump the size of EC2 AMIs up to 5 GB.

The FreeBSD base system continues to expand. 4GB is now insufficient;
we passed 3 GB in May 2019; we passed 2 GB in August 2017. Over half
of the disk space used is in /usr/lib/debug/.

Without this change, instances boot but are unusable, since the first
thing which breaks when VM filesystems are too small is the "pkg install"
in the VM building process.