modified checkout PRs branch step to support the case when PRs branch…

… is from another repository

Signed-off-by: Vladyslav Diachenko <vlad.diachenko@grafana.com>

.github/workflows/helm-loki-ci.yml

@@ -38,6 +38,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           ref: ${{ github.event.pull_request.head.ref }}
+          repository: ${{ github.event.pull_request.head.repo.full_name }}
           path: ${{ github.workspace }}/pr
 
       - name: Render Helm chart for each scenario in the base branch