Use the datestamp parameter (stored in NOW) instead of the newly added BUILD_ID.

debian/changelog

@@ -1,7 +1,7 @@
 livecd-rootfs (2.635) UNRELEASED; urgency=medium
 
-  * Support generating a .disk/info file via ubuntu-image if BUILD_ID is
-    passed from cdimage to the builder.
+  * Support generating a .disk/info file via ubuntu-image from the passed-in
+    datestamp parameter (using the $NOW environment variable).
 
  -- Łukasz 'sil2100' Zemczak <lukasz.zemczak@ubuntu.com>  Fri, 13 Dec 2019 18:12:12 +0100
 

live-build/auto/config

@@ -342,10 +342,10 @@ case $IMAGEFORMAT in
 				;;
 		esac
 
-		# If we have a build_id coming from cdimage, use that to populate
+		# If we have a datestamp coming from cdimage, use that to populate
 		# .disk/info on the target image
-		if [ -n "$BUILD_ID" ]; then
-			echo "$BUILD_ID" > config/disk-info
+		if [ -n "$NOW" ]; then
+			echo "$NOW" > config/disk-info
 			UBUNTU_IMAGE_ARGS="$UBUNTU_IMAGE_ARGS --disk-info config/disk-info"
 		fi