This uses the new size-change-report.sh script to print out some
information about largest files being added/removed and files with
greatest increase/decrease in file size between two versions of the
image.
This relies on flatcar_production_image_initrd_contents.txt being
uploaded to the server. It also exports the WITHWTD environment
variable with a value 1, which will make the package-diff script to
try out the wtd contents file variant first.
To review the image changes and the changelog more easily and in case
of fixes, iterate over it without rebuilding the image, move this logic
to its own file where a new job could call it.
