detect PR from GITHUB_HEAD_REF instead of github.event.pull_request.head.sha

drop the `checkout.with.ref` requirement in favour of CML auto-detecting the PR from the `$GITHUB_HEAD_REF` [env var](https://docs.github.com/en/actions/learn-github-actions/environment-variables)?

```diff
on: pull_request
...
- uses: actions/checkout@v3
- # quite clunky and might not be necessary
- with:
- ref: ${{ github.event.pull_request.head.sha }}
+ # auto-detect PR from $GITHUB_HEAD_REF
- run: cml comment ... --pr
```

in such cases, we can also print a warning message: "`current_sha != GITHUB_HEAD_REF` (so could result in e.g. confusing `dvc diff` etc.) so consider using `ref: ${{ github.event.pull_request.head.sha }}`"