scss: include clearfix on blocks zebra rows (forgotten from #45369)

static/includes/wcs/_bulk.scss

@@ -772,6 +772,9 @@ div.section.foldable {
 .pk-blocks-zebra {
 	// add a gray background for even numbered blocks, but that means odd
 	// childs when the first child is an error.
+	.BlockSubWidget {
+		@include clearfix();
+	}
 	.error ~ .BlockSubWidget:nth-child(odd),
 	.BlockSubWidget:first-child ~ .BlockSubWidget:nth-child(even) {
 		background: hsla(0, 0%, 0%, 0.05);