Pass variables to child pipelines

This change passes some variables to the trusted pipeline so it can be aware of where it was launched from. The initial variable sets:

• TRUSTED_SOURCE_BRANCH_NAME: The original branch name (without the trusted- prefix)
• TRUSTED_SOURCE_MR_IID: The merge request ID that triggered the test

As an initial set, this includes the source branch name and the MR IID. Other variables can be added as needed.

I wasn't able to get the local builds working, in part because the logic of our pipelines relies on coming from an MR. I think this is leftover from the original development of this script, and isn't a viable way to use it anymore. Therefore, this is gone and job tokens are required.

I also updated some of the underlying technology on this branch -- TaskLemon, Docker, and the GitLab CI language -- just to keep it fresh since this project doesn't get frequent changes.