Use a public-facing mirror URL in the final buildd /etc/apt/sources.list.

live-build/auto/config

@@ -677,7 +677,7 @@ case $SUBPROJECT in
 		OPTS="${OPTS:+$OPTS }--apt-secure false"
 		OPTS="${OPTS:+$OPTS }--linux-packages=none --initramfs=none"
 		INITRAMFS_COMPRESSION=none
-		OPTS="${OPTS:+$OPTS }--mirror-binary ${MIRROR}"
+		OPTS="${OPTS:+$OPTS }--parent-mirror-binary ${MIRROR}"
 		# XXX cjwatson 2018-04-27: We need to work out how to make
 		# this conditional so that we can do things like building
 		# buildd chroots with -updates.  This probably involves

live-build/buildd/hooks/mirror.binary

@@ -0,0 +1,12 @@
+#! /bin/sh
+set -e
+
+. config/bootstrap
+
+# Use a public-facing mirror URL, for the benefit of
+# sbuild-launchpad-chroot.  We deliberately do this only after live-build
+# has run "apt-get update" for the last time, in order that
+# /var/lib/apt/lists/ has suitable cached Packages files; this speeds up
+# builds on buildds.
+sed "s,${LB_PARENT_MIRROR_BINARY},${LB_MIRROR_BINARY},g" \
+	binary/etc/apt/sources.list