fix: Checkout PR HEAD commit instead of merge commit

[shenxianpeng]

self test commit-check action

[shenxianpeng]

It should be display failure summary in https://github.com/commit-check/commit-check-action/actions/runs/11402894009?pr=72

but right now it displays success.

[shenxianpeng]

It should be display failure summary in https://github.com/commit-check/commit-check-action/actions/runs/11402894009?pr=72

but right now it displays success.

Checkout pull request HEAD commit instead of merge commit can fix about issue

- uses: actions/checkout@v4
  with:
    ref: ${{ github.event.pull_request.head.sha }}

https://github.com/actions/checkout?tab=readme-ov-file#checkout-pull-request-head-commit-instead-of-merge-commit