build_linux: don't build the sample plugin

It shouldn't be included in releases.

build_linux.sh

@@ -20,7 +20,7 @@ export GO="${GO:-go}"
 mkdir -p "${PWD}/bin"
 
 echo "Building plugins ${GOOS}"
-PLUGINS="plugins/meta/* plugins/main/* plugins/ipam/* plugins/sample"
+PLUGINS="plugins/meta/* plugins/main/* plugins/ipam/*"
 for d in $PLUGINS; do
 	if [ -d "$d" ]; then
 		plugin="$(basename "$d")"