Merge branch 'fix_slack_notification_build' into 'master'

Don't setup DB for slack notification build

## What does this MR do?
Fix error with build related to slack notification: https://gitlab.com/gitlab-org/gitlab-ee/builds/2834026

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

## What are the relevant issue numbers?

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [ ] [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)

See merge request !5661

.gitlab-ci.yml

@@ -252,6 +252,9 @@ coverage:
 
 notify:slack:
   stage: post-test
+  variables:
+    USE_DB: "false"
+    USE_BUNDLE_INSTALL: "false"
   script:
     - ./scripts/notify_slack.sh "#builds" "Build on \`$CI_BUILD_REF_NAME\` failed! Commit \`$(git log -1 --oneline)\` See <https://gitlab.com/gitlab-org/$(basename "$PWD")/commit/"$CI_BUILD_REF"/builds>"
   when: on_failure