jenkins.sh: start with apt-get update

Without a fresh cache of packages, it's possible that we can run into
installation errors when trying to fetch packages. This adds an up-front
apt-get update to fix that.
zerovm-cli/jenkins.sh: start with apt-get update

Without a fresh cache of packages, it's possible that we can run
into installation errors when trying to fetch packages. This adds an
up-front apt-get update to fix that.