cml pr: is racy, generates a PR for different submissions

Sorry for the title. I don't have a good way to express what went wrong here.

This happened twice in `ldb-hackathon` for me where it generated a PR, where the contents were quite different from the result that I was expecting. In fact, the same contents were already generated before my workflow even ran. I am not quite sure here why cml is creating a PR with content that is already in the repo (even if the `git pull` is done before `cml pr` to try to keep the repo in sync).

1. `13:53 GMT`: This PR [#94](https://github.com/iterative/ldb-hackathon/pull/94/files) was published.
2. [`13:55 GMT`](https://github.com/iterative/ldb-hackathon/actions/runs/2847051140): Triggered workflow.
3. [`13:57 GMT`](https://github.com/iterative/ldb-hackathon/runs/7807546288?check_suite_focus=true#step:1:1): Started "evaluate" job/`train` step.
4. [`16:47:50 GMT`](https://github.com/iterative/ldb-hackathon/runs/7807546288?check_suite_focus=true#step:13:28733): Finished training
5. [`16:47:58 GMT`](https://github.com/iterative/ldb-hackathon/runs/7807546288?check_suite_focus=true#step:15:13): Created a PR [#106](https://github.com/iterative/ldb-hackathon/pull/106/files) with identical contents to [#94](https://github.com/iterative/ldb-hackathon/pull/94/files).

```console
Run cml pr --rebase 'submissions/*.json'
cml pr --rebase 'submissions/*.json'
shell: sh -e {0}
env:
AWS_DEFAULT_REGION: us-east-1
AWS_REGION: us-east-1
AWS_ACCESS_KEY_ID: ***
AWS_SECRET_ACCESS_KEY: ***
AWS_SESSION_TOKEN: ***
REPO_TOKEN: ***
{"level":"warn","message":"Failed to enable auto-merge: Enable the feature in your repository settings: https://github.com/iterative/ldb-hackathon/settings#merge_types_auto_merge. Trying to merge immediately..."}
https://github.com/iterative/ldb-hackathon/pull/106(https://github.com/iterative/ldb-hackathon/runs/7807546288?check_suite_focus=true#step:15:6)
```

Also I am a bit surprised to see `cml pr` using same branch name for submissions, which may likely be the issue here.
The commits are same and is from earlier time.