The build_manifest.py script

Adds a script that is able to determine the repo that the CI needs to build and generate a manifest from that.

Sadly turns out some assumption we had about CI_JOB_TOKEN being able to be used to authenticate to query the gitlab API are not true. Thus the gitlab Group API currently does not work. I am tempted to just drop the relevant part of code for now so we can start using it and implement a work around later.

cc @bilboed, @thiblahute