Provide CI_API_V4_URL environment variable

Internal Slack Discussion: https://gitlab.slack.com/archives/C02PF508L/p1543416106950600

With https://gitlab.com/gitlab-org/gitlab-ce/issues/50319, we changed the scope of PAT and can no longer do certain actions which were part of our (Distribution) internal workflow using them. This required us to change the logic to use API for those tasks - for example downloading a build artifact - omnibus-gitlab!2887 (merged) .

However, this caused another issue - we now have to hard code the URL to API - which makes it difficult to have separate workflows on gitlab.com and dev.gitlab.org without having conditional around them.

Proposal

Provide CI_API_V4_URL environment variable which points to the V4 API end point - /api/v4.

cc @jlenny @SteveAzz @twk3