CI: Add default workflow to avoid duplicate pipelines
This MR adds a default workflow: to avoid duplicate pipelines on MRs like https://gitlab.com/gitlab-org/tanuki_emoji/-/merge_requests/30/pipelines.
See https://gitlab.com/gitlab-org/tanuki_emoji/-/issues/23#note_902320617.
Fixes https://gitlab.com/gitlab-org/tanuki_emoji/-/issues/23
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Edited by Peter Leitzen