Image Customizer: Fix ISO customization. (microsoft#10885)

In change microsoft#10789, the `imagecustomizerapi.FileSystem` type had an internal field added called `PartitionId` which is filled in by the API validity checks.

ISO to ISO customization supports cracking open the squashfs file and modifying the Live-OS. However, to do this, a temporary OS image must be created. This creation logic uses the `imagecustomizerapi.FileSystem` type but the code wasn't updated to ensure the `PartitionId` field has a value.

toolkit/tools/pkg/imagecustomizerlib/liveosisobuilder.go

@@ -1444,16 +1444,18 @@ func (b *LiveOSIsoBuilder) createWriteableImageFromSquashfs(buildDir, rawImageFi
 
 	fileSystemConfigs := []imagecustomizerapi.FileSystem{
 		{
-			DeviceId: "esp",
-			Type:     imagecustomizerapi.FileSystemTypeFat32,
+			DeviceId:    "esp",
+			PartitionId: "esp",
+			Type:        imagecustomizerapi.FileSystemTypeFat32,
 			MountPoint: &imagecustomizerapi.MountPoint{
 				Path:    "/boot/efi",
 				Options: "umask=0077",
 			},
 		},
 		{
-			DeviceId: "rootfs",
-			Type:     imagecustomizerapi.FileSystemTypeExt4,
+			DeviceId:    "rootfs",
+			PartitionId: "rootfs",
+			Type:        imagecustomizerapi.FileSystemTypeExt4,
 			MountPoint: &imagecustomizerapi.MountPoint{
 				Path: "/",
 			},