Retry the cowbuilder create operation on failure

Sometimes while the debian repositories are updating there are
sporadic signature failures.  It's annoying to have these break the
build, and the only thing to do is to retry, so we'll retry here
automatically.  We were already retrying on the update operation that
can fail in a similar manner.

debian/util.sh

@@ -295,7 +295,10 @@ build_debs () {
     }
     if ! [ -d $cow_img ]; then
       announce "Creating base $distro-$arch image..."
-      cow --create
+      local x=5
+      while ! cow --create; do
+        [ $x -lt 1 ] && break; sleep 60; x=$((x-1))
+      done
     fi
     announce "Updating base $distro-$arch image..."
     local x=5