fix clean build again, use cache if actually there

whit dropping this in 0cec05b ("build initramfs: download packages
into per-flavor dirs"), the pre-caching was made effectively useless,
and building without internet during the build-step failed.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

src/build_initramfs.sh

@@ -12,6 +12,7 @@ mkdir -p "$BUILDDIR"
 if [ -d pkgs ]; then
     echo "copying package cache into build-dir"
     cp -a pkgs "$BUILDDIR/pkgs"
+    NO_DOWNLOAD="1"
 fi
 cd "$BUILDDIR"
 mkdir "$ROOT"