fix: Mark grub-pc-bin as manual install to avoid autoremoval (LP: #2045418)

With amd64 build target calling `update-grub` no longer works and fails build with error

```
grub-install: error: /usr/lib/grub/i386-pc/modinfo.sh doesn't exist. Please specify --target or --directory.
```

`/usr/lib/grub/i386-pc/modinfo.sh` is part of the grub-pc-bin package.

We are seeing this now on xenial as grub-pc-bin is now removed during build as part of fixing
bug https://bugs.launchpad.net/ubuntu/+source/livecd-rootfs/+bug/2036195.

To resolve this we mark grub-pc-bin as manually installed so it is not autoremoved.

As we are running grub-install ourselves for a specific target we need to make sure to install that
target first which means this will not be solved by changing dependencies of the grub packages.

debian/changelog

@@ -1,3 +1,9 @@
+livecd-rootfs (2.408.70) xenial; urgency=medium
+
+  * Mark grub-pc-bin as manual install to avoid autoremoval (LP: #2045418)
+
+ -- Philip Roche <phil.roche@canonical.com>  Mon, 04 Dec 2023 12:39:49 +0000
+
 livecd-rootfs (2.408.69) xenial; urgency=medium
 
   [Steve Langasek]

live-build/ubuntu-cpc/hooks.d/base/disk-image-uefi.binary

@@ -86,6 +86,10 @@ install_grub() {
             efi_target=arm64-efi
             ;;
         amd64)
+            # LP#2045418 - grub-pc-bin is installed as a dependency of grub-pc and is required for grub-install.
+            # However, when grub-efi-amd64 is installed, grub-pc is removed and grub-pc-bin is marked for autoremoval.
+            # This causes grub-install to fail. To avoid this we mark grub-pc-bin as manually installed.
+            chroot mountpoint apt-mark manual grub-pc-bin
             chroot mountpoint apt-get install -qqy grub-efi-amd64-signed grub-efi-amd64 shim-signed
             grub_modules="multiboot serial usb usb_keyboard"
             efi_target=x86_64-efi