Use a snap cohort key for consistent parallel builds (LP: #1866159)

debian/changelog

@@ -1,3 +1,9 @@
+livecd-rootfs (2.408.57) xenial; urgency=medium
+
+  * Use a snap cohort key for consistent parallel builds (LP: #1866159)
+
+ -- Robert C Jennings <robert.jennings@canonical.com>  Fri, 21 Feb 2020 21:32:37 -0600
+
 livecd-rootfs (2.408.56) xenial; urgency=medium
 
   * Backport enabling of adding extra snaps to ubuntu-core images via the

live-build/functions

@@ -367,6 +367,7 @@ _snap_preseed() {
         set -x;
         cd /var/lib/snapd/seed;
         SNAPPY_STORE_NO_CDN=1 snap download \
+            --cohort="${COHORT_KEY:-}" \
             --channel=$CHANNEL \"$SNAP_NAME\"" || snap_download_failed=1
     if [ $snap_download_failed = 1 ] ; then
         echo "If the channel ($CHANNEL) includes '*/ubuntu-##.##' track per "