misc: adapt "display as PNG" to find canvas in new HTML tags (#36006)

bijoe/templates/bijoe/cube_chart.html

@@ -17,7 +17,7 @@
                draw_barchart(canvas, drilldown, [measure], measure.name, data);
                /* Allow getting a PNG without using 'Save image as' */
                $(".bijoe-png-button").on('click', function() {
-                  this.href = toDataURL($(this).next("canvas")[0], "graph.png");
+                  this.href = toDataURL($(this).next().find("canvas")[0], "graph.png");
                })
             }
          }, 500 * {{ forloop.counter }});