Prevents the load more button from being cut off in some themes (#14669)

Adds margin bottom to the primary area of the overlay, which solves this problem but is also good practice to ensure search results are readable until the very last one.

modules/search/instant-search/components/search-results.scss

@@ -35,6 +35,7 @@
 
 .jetpack-instant-search__search-results-primary {
 	flex: 5;
+	margin-bottom: 6em;
 
 	@include break-medium() {
 		margin-right: 4em;