/usr/share/live/images/gnome-desktop/auto/build is in live-image-gnome-desktop 4.0~a5-1.
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 | #!/bin/sh
set -e
lb build noauto "${@}" 2>&1 | tee build.log
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