Use build num instead of workflow id for circleci

[josegonzalez]

:zap: Summary

This MR changes the coveralls build id to use CIRCLE_BUILD_NUM instead of CIRCLE_WORKFLOW_ID for CircleCI builds.

:ballot_box_with_check: Checklist

• [x] Add specs

[mrexox]

Hey! As long as the docs say, CIRCLE_BUILD_NUM is unique for each job. But workflow can contain more than one job, and coveralls can be used in parallel flow. In that case we need a more global identifier.

Do you feel CIRCLE_WORKFLOW_ID is inconvenient because of its length?

[josegonzalez]

I was just rebasing the PR so it worked on the newer codebase. That said, if it's not desired, its not desired :)