Force rebuild of plugins when buck is updated

Change-Id: Iad43824624fbe702bf96248510ce04ecb46de7be
diff --git a/tools/build/onos-buck b/tools/build/onos-buck
index 0ada396..123a9b6 100755
--- a/tools/build/onos-buck
+++ b/tools/build/onos-buck
@@ -31,6 +31,7 @@
     chmod 444 .buck_version
     rm -rf ./buck-out
     printf "Successfully downloaded Buck to $ONOS_ROOT/bin/buck\n\n"
+    ONOS_BUILD_PLUGIN="true"
 fi
 popd > /dev/null