[Merge] ~vorlon/livecd-rootfs:subarch-manifest into livecd-rootfs:ubuntu/master

[Philip Roche]

Review: Needs Fixing

I believe this will fail for riscv64 cloud images like https://launchpad.net/~cloudware/+livefs/ubuntu/noble/cpc/+build/539365 when no SUBARCH is defined. 
-- 
https://code.launchpad.net/~vorlon/livecd-rootfs/+git/livecd-rootfs/+merge/451944
Your team Ubuntu Core Development Team is subscribed to branch livecd-rootfs:ubuntu/master.