/usr/share/ubiquity/update-apt-cache is in ubiquity 18.04.14.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 | #!/bin/sh
set -e
mkdir -p /target/var/cache/apt/archives
mkdir -p /target/var/cache/apt/archives/partial
apt-get update
apt-get -d -y --force-yes -q -o Dir::Cache=/target/var/cache/apt upgrade
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