Choosing CI config path when running pipelines
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### Summary
I want to be able to choose a CI config path when running a pipeline.
Scenarios;
> The use-case for me is to have several scheduled-pipelines that runs different pipelines, from a single repository.
> I also need to inject the filename at pipeline level.
> i just ran into this issue when trying to trigger a pipeline via the API, ie curl with `--form variables[CI_CONFIG_PATH]=gitlab/ci.yml`
issue