manibuilder: Fix Ubuntu Noble Numbat (24.04) for amd64

Some parts of the Dockerfile.ubuntu accidentally broke the apt setup for
amd64. Check for the newer `/etc/apt/sources.list.d/ubuntu.sources', and
skip the old hacks if it exists.

Change-Id: Iae42ba548e5ffa228922bd060a38450e969e4405
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.sourcearcade.org/c/flashprog/+/273
diff --git a/util/manibuilder/Dockerfile.ubuntu b/util/manibuilder/Dockerfile.ubuntu
index b9b818f..326f17f 100644
--- a/util/manibuilder/Dockerfile.ubuntu
+++ b/util/manibuilder/Dockerfile.ubuntu
@@ -2,7 +2,9 @@
 
 RUN \
 	useradd -p locked -m mani && \
-	if grep -q main /etc/apt/sources.list; then \
+	if [ -f /etc/apt/sources.list.d/ubuntu.sources ]; then \
+		true; \
+	elif grep -q main /etc/apt/sources.list; then \
 		if ! grep -q universe /etc/apt/sources.list; then \
 			sed -i -e 's/ main$/ main universe/' \
 				/etc/apt/sources.list || exit 1; \