added CI_MERGE_REQUEST_ID to build env if the build is for a merge request

What does this MR do?

Adds CI_MERGE_REQUEST_ID as an environment variable for builds

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

Not that I know of

Why was this MR needed?

I'm working with others editing the danger gem - https://github.com/danger/danger - to add support for GitLab

What are the relevant issue numbers?

None that I know of

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together