Use a less memory-intensive sourcemap when running in CI

What does this MR do?

After changing the webpack config slightly, the jobs in EE started crashing with errors like:

  55) Projects::TagsController GET index returns releases matching those tags
      Failure/Error: IO.popen(update_refs, "w") {|io| io.write(instructions) }

      Errno::ENOMEM:
        Cannot allocate memory - /usr/bin/git
      # ./spec/support/test_env.rb:217:in `popen'
      # ./spec/support/test_env.rb:217:in `block in set_repo_refs'
      # ./spec/support/test_env.rb:223:in `block in set_repo_refs'
      # ...

Apparently the use of our kitchen-sink eval inline-source-map was taking up a gigantic amount of memory in PhantomJS and causing some other processes to fail. This was only happening in EE presumably because of the additional processes that it runs (LDAP, elastic search, etc).

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

should be pretty simple

Why was this MR needed?

EE master will break once the changes in !10437 (merged) make it from CE to EE. This will prevent things from blowing up.

Screenshots (if relevant)

n/a

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
• 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

What are the relevant issue numbers?