kernel-install/60-ukify: do not rebuild existing UKIs

kernel-install/60-ukify: do not rebuild existing UKIs

In case kernel-install is called to install what's already a UKI, 60-ukify
must be skipped. E.g. when distro shipped and signed UKI is being installed,
it is counter-productive to try to rebuild it with ukify. Also, the existing
script is not ready to handle UKIs with embedded initramfs.

Note, it is already possible to disable 60-ukify by setting
KERNEL_INSTALL_UKI_GENERATOR to something but in case it is not set, 60-ukify
assumes it should run.