Merge branch 'invisible-scroll-controls-on-build-page' into 'master'

Fixed invisible scroll controlls on build page on iPhone

## What does this MR do?
Fixes invisible scroll controls on build page on iPhone

## Are there points in the code the reviewer needs to double check?
No.

## Why was this MR needed?
Because it's not correct behavior.

## Screenshots (if relevant)

| Before | After |
| -------- | -------- |
| ![IMG_0424](/uploads/19a6b498ad8be265e52619b9e7b71975/IMG_0424.PNG)   | ![IMG_0425](/uploads/3f7a7e00475a2e4c78e77e3bc76ac9f3/IMG_0425.PNG)  |

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
Not found

See merge request !6125

CHANGELOG

@@ -54,6 +54,7 @@ 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%;