Handle UKI/Type1 boot entries properly

Currently in this function https://github.com/bootc-dev/bootc/blob/ca9f93c33f0c1e463035788f4231a3f294dafeea/crates/lib/src/bootc_composefs/boot.rs#L1250 we're relying on `fs.transform_for_boot(&repo)` to return boot entries in some particular order viz

1. Type1 entries in `/boot/loader/entries` (almost never used)
2. UKIs
3. Type1 Kernel + Initrd

We have been getting away with this even though our UKI images contain both UKI + (Kernel + Initrd) because the UKI appear earlier in the list. We should handle this more carefully