`install to-disk` should not create a BIOS partition for composefs+UKI installations by default

When installing an image using `bootc install to-disk` and selecting the composefs backend and using a UKI, bootc should not create a BIOS partition as it will not be used.

We may want to add an option to still add one if the system is expected or is immediately degraded into a kernel + initrd installation without a UKI.

Note that in Fedora CoreOS we may still choose to create those for compatibility but we will be using `to-filesystem`.

Start by tracing the `bootc install to-disk` partition-layout path and the conditions for composefs with a UKI. Check how existing installation tests cover BIOS partitions; done means this combination no longer creates a BIOS partition by default, while any compatibility option or behavior remains explicitly defined and tested.

Scritto dal modello di indicizzazione a partire dal testo della issue.