Add casper to minimal.standard.live seed

Without casper in the minimal.standard.live seed for flavors using the
new ubuntu-desktop-installer (or derivitives thereof), casper cannot
create a live user. Without this live user, Ubuntu Studio has been
experiencing the inability to login automatically from either the GUI or
manually from a TTY. This leaves the boot at a black screen with a mouse
cursor. This commit is an attempt to avoid the same situation. Previous
assessments of omitting casper from this line appear to have been
incorrect.

live-build/auto/config

@@ -805,7 +805,7 @@ case $PROJECT in
 		add_task minimal minimal standard edubuntu-desktop-gnome-minimal
 		add_task minimal.standard edubuntu-desktop-gnome
 		add_task minimal.standard.live edubuntu-live
-		add_package minimal.standard.live linux-$KERNEL_FLAVOURS
+		add_package minimal.standard.live linux-$KERNEL_FLAVOURS casper
 		cat <<-EOF > config/minimal.catalog-in.yaml
 			name: "Edubuntu Desktop (minimized)"
 			description: >-