fix(install.d): do not create initramfs if the supplied image is UKI

fix(install.d): do not create initramfs if the supplied image is UKI

When the supplied kernel image is a UKI, there's no point in creating
initramfs as the UKI has it built-in already. This is the situation when
dracut.install is called for a distro shipped UKI.

Note, KERNEL_INSTALL_IMAGE_TYPE == "uki" is different from
KERNEL_INSTALL_LAYOUT == "uki", the later can be used to create UKI upon
installing a standard kernel image.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>