Fix unbootable installs: lay the kernel into the target and own GRUB

archiso keeps vmlinuz/initramfs in the ISO boot dir, not the squashfs, so
unpackfs lays down an empty /boot. The chroot's mkinitcpio/grub-mkconfig had
nothing to work with and the ESP ended up empty (firmware found no bootloader).

- shellprocess@kernel (dontChroot) copies the live kernel into the target
  /boot before the bootloader step
- post-install.sh now runs grub-install itself, including a --removable pass
  so firmware with no NVRAM entry still boots via EFI/BOOT/BOOTX64.EFI

iso/airootfs/etc/calamares/settings.conf

@@ -1,6 +1,13 @@
 ---
 modules-search: [/etc/calamares/modules, /usr/lib/calamares/modules]
 
+# Second shellprocess instance: copies the live kernel into the target /boot
+# (archiso keeps it out of the squashfs) before the bootloader step runs.
+instances:
+- id: kernel
+  module: shellprocess
+  config: shellprocess-kernel.conf
+
 sequence:
   - show:
     - welcome
@@ -22,6 +29,7 @@ sequence:
     - networkcfg
     - hwclock
     - packages
+    - shellprocess@kernel
     - bootloader
     - shellprocess
     - umount