Fixed invisible scroll controlls on build page on iPhone

CHANGELOG

@@ -50,6 +50,7 @@ v 8.12.0 (unreleased)
   - Adds response mime type to transaction metric action when it's not HTML
   - Fix hover leading space bug in pipeline graph !5980
   - User can edit closed MR with deleted fork (Katarzyna Kobierska Ula Budziszewska) !5496
+  - Fixed invisible scroll controls on build page on iPhone
 v 8.11.4 (unreleased)
   - Fix broken gitlab:backup:restore because of bad permissions on repo storage !6098 (Dirk Hörner)

app/assets/stylesheets/pages/builds.scss

@@ -36,6 +36,7 @@
     &.affix {
       right: 30px;
       bottom: 15px;
+      z-index: 1;
       @media (min-width: $screen-md-min) {
         right: 26%;