Add explicit display block to .search form

- This was removed from application.scss in
  [this MR][1], but the startup CSS still had
  the explicit display: block

[1]: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/61753

app/assets/stylesheets/pages/search.scss

@@ -45,6 +45,7 @@ input[type='checkbox']:hover {
   margin: 0 8px;
 
   form {
+    display: block;
     margin: 0;
     padding: 4px;
     width: $search-input-width;