build: checkout full repo for checkpatch

checkpatch likes to verify git references. Thus we need to checkout the
full repo to get this working.

Signed-off-by: Daniel Wagner <[email protected]>

Author: dwsuse

.github/workflows/checkpatch.yml

@@ -10,6 +10,6 @@ jobs:
     - uses: actions/checkout@v4
       with:
         ref: ${{ github.event.pull_request.head.sha }}
-        fetch-depth: ${{ env.PR_FETCH_DEPTH }}
+        fetch-depth: 0
     - name: Run checkpatch review
       uses: webispy/checkpatch-action@v9