Temporarily disable automatic merging

It appears that for some jobs
(https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/129705025 for example)
we process a range of commits far greater than necessary, which can lead
to commits getting reverted that were merged a long time ago.

[ci skip]

.gitlab-ci.yml

@@ -1221,6 +1221,7 @@ merge:master:
       - $MERGE_TRAIN_SSH_PUBLIC_KEY
       - $MERGE_TRAIN_SSH_PRIVATE_KEY
       - $MERGE_TRAIN_API_TOKEN
+      - $MERGE_FORCE_ENABLE
   script:
     - scripts/merge-train
   cache: