Changes in shim-signed packaging and breakages in UC20 secured grade model images on amd64

[snowsky]

@xnox

As I said Ubuntu 20.04 (focal / archive.ubuntu.com) and Ubuntu Core 20 (pc gadget / core-specific PPAs) use different shims signed with different keys.

Hence you must use shim & grub & related efi binaries from the pc gadget.

Thanks for the details. We found there is an update for Ubuntu 20/22 on the secure boot key, https://blog.jak-linux.org/2023/02/01/ubuntu-key-rotation/. Will this rotation update also apply for Ubuntu Core 20/22?