Install 'yarn' globally to avoid PATH issues

What does this MR do?

Alternative to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13231

Fixes the following error in the installation guide:

$ sudo -u git -H yarn install --production --pure-lockfile
sudo: yarn: command not found

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

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

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