Fix race condition when previewing docs
What does this MR do?
Cancel the pipeline that gets created the first time the remote branch is created in order not to overwrite the one we trigger afterwards. For more details, see https://gitlab.com/gitlab-com/gitlab-docs/issues/154.
This can only be tested when using a new branch for the very first time.
How to test this
I tested with a new MR https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4839, and you can see the remote environment having only one deployment, the one we want https://gitlab.com/gitlab-com/gitlab-docs/environments/238836.
Closes gitlab-com/gitlab-docs#154, https://gitlab.com/gitlab-org/gitlab-ce/issues/41024