Comment 4 for bug 1872163

Here is *a* fix/workaround that I'd expect to work for existing releases and focal

$ diff -u /etc/schroot/setup.d/90apt-sources.dist /etc/schroot/setup.d/90apt-sources
--- /etc/schroot/setup.d/90apt-sources.dist 2020-04-17 14:26:50.510749407 -0400
+++ /etc/schroot/setup.d/90apt-sources 2020-04-17 14:27:40.814787763 -0400
@@ -113,6 +113,13 @@
         APT_COMPONENTS="$comps"
     fi

+ # LAUNCHPAD_SERIES comes from schroot config 'launchpad.series'
+ if [ -n "$LAUNCHPAD_SERIES" ] &&
+ [ -n "$APT_COMPONENTS" -o -z "$APT_POCKETS" ]; then
+ echo "Setting one-line sources.list (LP: #1873507)"
+ echo "deb http://archive.ubuntu.com/ubuntu/ ${LAUNCHPAD_SERIES} main restricted universe multiverse" > "$sources"
+ fi
+
     if [ -n "$APT_POCKETS" ]; then
         echo "setting apt pockets to 'release $APT_POCKETS' in sources.list"
         awk -v pockets="$APT_POCKETS" '