sudo apt install libjxl-tools
djxl flag.jxl flag.png
xdg-open flag.png
Edit the top of the original image (flag bits) so that the image verifies the flag as correct... however weird it sounds.
I would recommend an editor for this task, but as far as I know, nothing really supports JXL enough... so hexeditor, I guess?
Spoilers in the challenge directory.