Explicitly install apt-transport-https and ca-certificates

Raspbian buster is old enough that this isn't included by default, but it looks
like the mirror redirector now started redirecting to HTTPS mirrors. Thus,
install it so this has a chance to actually work.

Makefile

@@ -25,7 +25,7 @@ all: build
 
 tarball: rootfs.tar.xz
 rootfs.tar.xz:
-	TARBALL="$@.tmp" ./make-image.sh
+	TARBALL="$@.tmp" ./make-image.sh debootstrap.sh --include=apt-transport-https,ca-certificates
 	mv "$@.tmp" "$@"
 
 qemu-arm-static: