fix: Use Ubuntu policy default channel "latest" for preseeded snaps (LP: #2061121)

Recent Mantic cloud image builds fail after default track for LXD change to "5.21" instead of "latest"

This changes preseed from "latest/" for all snaps and no longer assume that "latest/" is the default.

debian/changelog

@@ -1,3 +1,10 @@
+livecd-rootfs (23.10.59) UNRELEASED; urgency=medium
+
+  * Use Ubuntu policy default channel "latest" for preseeded snaps
+    (LP: #2061121)
+
+ -- Philip Roche <phil.roche@canonical.com>  Fri, 12 Apr 2024 12:37:29 +0100
+
 livecd-rootfs (23.10.58) mantic; urgency=medium
 
   [ Simon Poirier ]

live-build/functions

@@ -730,7 +730,7 @@ snap_preseed() {
                 CHANNEL=${2#*=}
                 ;;
             *)
-                CHANNEL="stable/ubuntu-$(release_ver)"
+                CHANNEL="latest/stable/ubuntu-$(release_ver)"
                 ;;
         esac
     fi