Cirrus: fix empty checkpoint directory detection

tools/cirrus_build_project.sh

@@ -65,7 +65,7 @@ fi
 if [[ "$SHOULD_BUILD" -eq 1 ]]; then
     echo "Building project..."
     # If rbm fails, we consider it a success as long as it saved a checkpoint.
-    ./rbm/rbm build "$PROJECT" --target "$CHANNEL" --target ncdns-"$OS"-"$ARCH" || ls ./tmp/interrupted_dirs/*
+    ./rbm/rbm build "$PROJECT" --target "$CHANNEL" --target ncdns-"$OS"-"$ARCH" || [ ! -z "$(ls -A ./tmp/interrupted_dirs/)" ]
 else
     #echo "This is a cache-only task, skipping build."
     echo "Skipping build."