Fix ubuntu-server-live images to generate initrd with casper UUID.

Whilst configs are sources, the variables are not exported into
the environment by default. Thus when casper hook is executed, it had
no idea that it should generate UUID.

live-build/ubuntu-server/hooks/033-kernel-bits.binary

@@ -55,7 +55,7 @@ for variant in $variants; do
 	if [ -n "$LB_INITRAMFS_COMPRESSION" ]; then
 		echo "COMPRESS=$LB_INITRAMFS_COMPRESSION" > "$KERNEL_BITS_ROOT"/etc/initramfs-tools/conf.d/livecd-rootfs.conf
 	fi
-	echo "CASPER_GENERATE_UUID=1" > "$KERNEL_BITS_ROOT"/etc/initramfs-tools/conf.d/casper.conf
+	echo "export CASPER_GENERATE_UUID=1" > "$KERNEL_BITS_ROOT"/etc/initramfs-tools/conf.d/casper.conf
 
 	# Add a hook to record which kernel was booted and mount the
 	# modules.squashfs created below.