auto/config: seed ubuntu-desktop when building hyperv image

Regression introduced in 38157b3748 when
desktop-preinstalled code branch was added, it dropped addint
ubuntu-desktop task. Instead it added ubuntu-desktop-raspi task, only
for the raspi subarch, which depends on ubuntu-desktop. But the hyperv
case, now ended up without ubuntu-desktop task.

It looks like introduction of "desktop-preinstalled" assumed, that it
is for raspi only, when in fact that code path now started to be used
for hyperv gallery image too.

(cherry picked from commit 5d9af7f3c5)

LP: #1901846

live-build/auto/config

@@ -601,7 +601,7 @@ case $PROJECT in
 				;;
 
 			desktop-preinstalled)
-				add_task install minimal standard
+				add_task install minimal standard ubuntu-desktop
 				if [ "$SUBARCH" = "raspi" ]; then
 					add_task install ubuntu-desktop-raspi
 				fi