pkg: fedora: Use the proper path to set the default kernel

grubby --set-default expects the path that GRUB will use to access
the kernel. This adds compatibility with /boot on a BTRFS subvolume.

pkg/fedora/kernel-surface/files/linux-surface-default-watchdog.sh

@@ -16,7 +16,7 @@ VMLINUX="$(echo "${KERNELS}" | tail -n1 | cut -d' ' -f2)"
 echo "${VMLINUX}"
 
 # update GRUB config
-grubby --set-default "${VMLINUX}"
+grubby --set-default "$(grub2-mkrelpath "${BOOTPATH}")"
 
 # Update timestamp for rEFInd
 # Ensure it's marked as latest across all kernels, not just surface ones