[bug] scm capture fails during Github PR workflow

### Describe the bug

Not strictly a conan bug, more of an oversight in the documentation but since the [Documentation on SCM capture](https://docs.conan.io/2/examples/tools/scm/git/capture_scm/git_capture_scm.html) explicitly uses a github workflow as illustration, it's probably worth reporting.

Expected Result
----------------------

A conanfile using the SCM capture feature should build during a workflow triggered by a Github PR event

Actual Result
------------------

Conanfile build fails with

```
fatal: unable to read tree (cf912fbe1bc5dee48e6117f9a34b60f59a51f578)
```

See [Logs](https://github.com/planetmarshall/conan-scm-capture/actions/runs/10850669544/job/30112656886) for failing action.

Workaround
----------------

Explicitly specify the pull request head ref when checking out the code

```yml
- name: Checkout
uses: actions/checkout@v4
with:
ssh-key: ${{ secrets.SSH_PRIVATE_KEY }}
ref: ${{ github.event.pull_request.head.sha }}
```

### How to reproduce it

Clone the repository at https://github.com/planetmarshall/conan-scm-capture.git and create a pull request for the `setup-ci` branch