fix(iso): Ship an archiso preset for linux-cachyos, the kernel we boot

linux.preset targeted the 'linux' package, which is not on the ISO, so
linux-cachyos generated its own default+fallback preset and the squashfs
carried an unused fallback initramfs. No package owns the preset file
(mkinitcpio's hook only creates one if missing), so this cannot conflict.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

airootfs/etc/mkinitcpio.d/linux-cachyos.preset

@@ -0,0 +1,11 @@
+# mkinitcpio preset file for the 'linux-cachyos' package on archiso
+# (the kernel the boot entries load). Not owned by any package: mkinitcpio's
+# install hook only generates a preset when none exists, so this one wins and
+# the image gets the archiso initramfs alone - no unused fallback image.
+
+PRESETS=('archiso')
+
+ALL_kver='/boot/vmlinuz-linux-cachyos'
+archiso_config='/etc/mkinitcpio.conf.d/archiso.conf'
+
+archiso_image="/boot/initramfs-linux-cachyos.img"

airootfs/etc/mkinitcpio.d/linux.preset

@@ -1,8 +0,0 @@
-# mkinitcpio preset file for the 'linux' package on archiso
-
-PRESETS=('archiso')
-
-ALL_kver='/boot/vmlinuz-linux'
-archiso_config='/etc/mkinitcpio.conf.d/archiso.conf'
-
-archiso_image="/boot/initramfs-linux.img"