Looks like this is an issue with my repo and I fixed it by adding a leaner boot_menu.yml file and keeping the setting for it inside gitlab-ci.yml -- i think one of the following approaches would be a good improvement:
Provide a default boot_menu.yml (which actually exists in isogenerator repo) and use it when the custom repo doesn't provide one
Better document it for what this file is for and how to properly change it (ex: ublue_variants is a keyword that should not be changed)
I'm trying to add isogenerator to my custom silverblue image but it fails with some errors: https://github.com/tunix/penguix/actions/runs/4846485642/jobs/8636094281