[CI] Fixed checkout commit for PR in case of pull_request_target (#6573)

Fixed checkout commit for PR in case of pull_request_target

Author: apstasen

.github/workflows/sycl_precommit.yml

@@ -25,6 +25,7 @@ jobs:
     steps:
     - uses: actions/checkout@v2
       with:
+        ref: ${{ github.event.pull_request.head.sha }}
         persist-credentials: false
         fetch-depth: 2
     - name: Run clang-format
@@ -46,6 +47,7 @@ jobs:
     uses: ./.github/workflows/sycl_linux_build_and_test.yml
     secrets: inherit
     with:
+      build_ref: ${{ github.event.pull_request.head.sha }}
       build_cache_root: "/__w/"
       build_cache_size: "8G"
       build_artifact_suffix: "default"