MT#62787 SW-RAID: install only to first RAID disk to not fail in non-EFI mode

Noted in daily-build-carrier-runner installation (see
https://jenkins.mgm.sipwise.com/job/daily-build-install-vm/148320/console):

| Installing for i386-pc platform.
| grub-install: error: unable to identify a filesystem in mduuid/ad9e241cd5c1093cd46de7932bc3fce1; safety check can't be performed.
| Error: failed to execute 'grub-install --no-floppy --target=i386-pc /dev/md0'.

One can run 'grub-install /dev/md0' without problems on EFI systems, as
this doesn't really touch the disks then. But in non-EFI systems, this
fails hard, because GRUB doesn't support direct installation to the
SW-RAID.

Instead we need to iterate over all the disks separately, being already
handled within stage swraidinstallgrub:

| +11:37:12 (deployment.sh:2457):  set_deploy_status swraidinstallgrub
| +11:37:12 (deployment.sh:102): set_deploy_status():  '[' -n swraidinstallgrub ']'
| +11:37:12 (deployment.sh:103): set_deploy_status():  echo swraidinstallgrub
| +11:37:12 (deployment.sh:2458):  for disk in "${SWRAID_DISK1}" "${SWRAID_DISK2}"
| +11:37:12 (deployment.sh:2459):  grml-chroot /mnt grub-install /dev/sda
| Writing /etc/debian_chroot ...
| Installing for i386-pc platform.
| Installation finished. No error reported.
| +11:37:12 (deployment.sh:2458):  for disk in "${SWRAID_DISK1}" "${SWRAID_DISK2}"
| +11:37:12 (deployment.sh:2459):  grml-chroot /mnt grub-install /dev/sdb
| Writing /etc/debian_chroot ...
| Installing for i386-pc platform.
| Installation finished. No error reported.
| +11:37:13 (deployment.sh:2462):  grml-chroot /mnt update-grub
| Writing /etc/debian_chroot ...
| Generating grub configuration file ...
| Found linux image: /boot/vmlinuz-6.12.33+deb13-amd64
| Found initrd image: /boot/initrd.img-6.12.33+deb13-amd64
| Adding boot menu entry for UEFI Firmware Settings ...
| done

So install GRUB only to the *first* SW-RAID disk.

Change-Id: I436e3deca37d68ef505d44fd24620028f2ded0f4
(cherry picked from commit 75b0f9274e)

templates/scripts/includes/deployment.sh

@@ -1728,7 +1728,7 @@ fi
 # disk information and set GRUB target
 if [[ "${SWRAID}" = "true" ]] ; then
   DISK_INFO="Software-RAID with $SWRAID_DISK1 $SWRAID_DISK2"
-  GRUB_DISK="${SWRAID_DEVICE}"
+  GRUB_DISK="/dev/${SWRAID_DISK1}"  # we iterate over the disks in swraidinstallgrub
 else
   DISK_INFO="/dev/$DISK"
   GRUB_DISK="/dev/$DISK"