[generic-efi] Some images don't get $ROOT_UUID defined

For some reason, currently unknown to me, some generic-efi images do not get a proper $ROOT_UUID set for Grub, while others, in the same CI run does. This results in the affected images to "Not able to find root" error after selecting the entry in Grub.

I have only seen this issue happen on the Github CI and I currently have no idea why.

Example:

In the last Dev image run today Minimal and Sway editions got the UUID, while the rest did not. There is nothing in the tools, as far as I can tell, that would prohibit the other editions from getting it.

I have verified that creating a kde-plasma (one of the failed ones) image on my local desktop, gets UUID just fine.