Don't delete previous iso images if building new ones fails

[zyli]

https://cdimage.debian.org/cdimage/weekly-live-builds/amd64/iso-hybrid/
Failed to build new iso images (except: kde and standard) - status as of 
2025-04-08.

Well - it happens.

But at the same time previous images were deleted.

My request:
in case of failure to build new iso images - shouldn't the previous ones 
be deleted.

Best regards