Wait for the pipeline to start before canceling it
What does this MR do?
Fix a failure of the docs review apps one more time...
./trigger-build-docs:45:in `create_remote_branch': undefined method `id' for nil:NilClass (NoMethodError)
from ./trigger-build-docs:117:in `<main>'
It appears there's a race condition (again) and the pipeline has not been created yet when we try to cancel it.
For context, the previous MR that attempted to fix this is https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17552, and the issue https://gitlab.com/gitlab-com/gitlab-docs/issues/154.