Pass custom env. variables specified from the github action to the gitlab pipeline

[mlubej]

Hi!

Very nice github action, we use it quite a bit, but noticed that we would like to split the behavior of the gitlab pipelines based on the trigger source of the github action. We would like to do this via a custom provided env variable, which could be specified during this job and sent over to gitlab, which is supported by the API: https://docs.gitlab.com/ee/ci/triggers/#pass-cicd-variables-in-the-api-call.

Thank you!