Initial support for automatic setup of GitLab CI credentials

[stertingen]

Slightly over-engineered alternative to

variables:
  AFTER_INIT_EMBED: "git config --global url.${CI_SERVER_PROTOCOL}://gitlab-ci-token:${CI_JOB_TOKEN}@${CI_SERVER_HOST}:${CI_SERVER_PORT}.insteadOf ${CI_SERVER_URL}"

[mathias-luedtke]

While it might not be necessary for Gitlab CI, such a credential helper might be good for Github access tokens.

[mathias-luedtke]

With #613, we could just prepare .gitconfig and .gitcredentials outside of the industrial_ci run (e.g. from gitlab.sh) and then just copy the credentials.
I don't think that passing the token as an environment variable is more secure than storing it in a file (in the context of CI tests!).

If we automate this in the CI-specific entry point, then it still works with run_ci..