Add the PR head as reference to the checkout action of the splitter job. (#981) (#1005)

This is a backport of PR #981 as merged into main (ec35c74).
SUMMARY
During the workflow checks of one of my previous pull requests, I ran into an error as result of a new integration test that was added in the main branch but which was not present in my pull request (https://github.com/ansible-collections/kubernetes.core/actions/runs/16787189478/job/47540604109). I'd find out that the reference for the repository checkout for the splitter job was not set and therefor it referenced to the main branch.
With this change, the head of the pull request is used as reference for the repository checkout action, in the splitter job. This way it will not fail when someone creates a PR that doesn't have newly added integration tests that were added to the main branch in the meantime. It also adds that newly added integration tests will be tested during the PR workflow checks already.
ISSUE TYPE

Bugfix Pull Request

COMPONENT NAME
bugfix in the workflow
ADDITIONAL INFORMATION
n/a

Reviewed-by: Bianca Henderson <beeankha@gmail.com>

.github/workflows/integration-tests.yaml

@@ -26,6 +26,7 @@ jobs:
         with:
           path: ${{ env.source_dir }}
           fetch-depth: "0"
+          ref: ${{ github.event.pull_request.head.sha }}
 
       - name: list changes for pull request
         id: splitter