github actions: Allow fork checkout for commit validation

Since we have external contributors we need process each of the commits
for faster reviews, validation that header information is correct and to
check the state of our tickets so that is actually tracked correctly
internally, on behalf of some external contributors.

Author: PlaidCat

.github/workflows/validate-kernel-commits.yml

@@ -18,7 +18,8 @@ jobs:
         uses: actions/checkout@v4
         with:
           fetch-depth: 0
-          ref: ${{ github.head_ref }}
+          repository: ${{ github.event.pull_request.head.repo.full_name }}
+          ref: ${{ github.event.pull_request.head.ref }}
 
       - name: Checkout base branch
         run: |