build: count files without a failing arithmetic command

Under set -e, ((n++)) with n at 0 evaluates to 0 and exits the script. Bash
3.2 on macOS ignores that case, so local builds passed while the release
runner's bash 5 stopped silently on the first file.

tools/build.sh

@@ -51,7 +51,7 @@ for filename in "${BUILD_FILES[@]}"; do
     if [ -f "$filepath" ]; then
         cat "$filepath" >> "$OUTPUT"
         echo "" >> "$OUTPUT"
-        ((file_count++))
+        file_count=$((file_count + 1))
     else
         echo "Warning: $filename not found, skipping" >&2
     fi