add push via CI_JOB_TOKEN

[pimperator]

Hi Guys,
gitlab has recently enabled the ability to push via CI_JOB_TOKEN. Details see here
Still the feature needs to be enabled and is in development but it is just a matter of time when it is stable.

Since any other personal, group or project access token might be not immediately revoked when a teammember leaves the company going back to CI_JOB_TOKEN is more safe. Also it works out of the box since it is available anyways.

For compatibility reasons the status quo shall remain. It should be either GL_TOKEN or GITLAB_TOKEN and if not defined CI_JOB_TOKEN.

[fgreinacher]

Thanks for reaching out.

We already track this via #156.

Feel free to join the discussion there!