Fix update-download-page.zig for latest Zig

See ziglang/zig#19844

.github/workflows/update-download-page.zig

@@ -95,5 +95,5 @@ fn render(
  line += 1;
  }
  }
- try std.fs.cwd().writeFile(out_file, buffer.items);
+ try std.fs.cwd().writeFile(.{ .sub_path = out_file, .data = buffer.items });
 }