also use apt_preferences(5) to install the newer debootstrap in the ubuntu nodes

Signed-off-by: Mattia Rizzolo <mattia@debian.org>

hosts/common-arm64/etc/apt/preferences.d/pbuilder

@@ -6,6 +6,12 @@ Package: munin-node munin-plugins-core munin-plugins-extra munin-common
 Pin: release o=Debian Backports
 Pin-Priority: 990
 
+# install a newer debootstrap as since 20180927 debootstrap 1.0.78+nmu1ubuntu1.6 cannot install sid anymore
+# also, pbuilder requires debootstrap >= 1.0.97
+Package: debootstrap
+Pin: release o=Debian Backports
+Pin-Priority: 990
+
 Package: *
 Pin: release o=Debian Backports
 Pin-Priority: -1

update_jdn.sh

@@ -323,10 +323,9 @@ if [ -f /etc/debian_version ] ; then
 			"
 		case $HOSTNAME in
 			codethink*)
-				# install debootstrap from stretch-backports on ubuntu nodes as since 20180927 debootstrap 1.0.78+nmu1ubuntu1.6 cannot install sid anymore
-				DEBS="$DEBS
-				debootstrap/stretch-backports
-				" ;;
+				# the apt_preferences(5) file will cause some packages to come
+				# from Debian backports
+				;
 			*)	# not available in Ubuntu 16.04:
 				DEBS="$DEBS
 				ripgrep