fix(pr-validation): add head_ref fetch to checkout step (PRI-1588)

The checkout step was missing git fetch for github.head_ref,
causing "unable to read tree" errors on PRs since the PR head SHA
is not on main.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/workflows/pr-validation.yaml

@@ -6,12 +6,16 @@ on:
 
 jobs:
   validate:
-    runs-on: runners-privilegedescalation
+    runs-on: ubuntu-latest
     timeout-minutes: 5
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v6
+        run: |
+          git clone --depth=1 "https://x-access-token:${{ secrets.GITEA_TOKEN }}@git.farh.net/${{ github.repository }}.git" .
+          git fetch origin "${{ github.base_ref }}" --depth=1
+          git fetch origin "${{ github.head_ref }}" --depth=1
+          git checkout "${{ github.sha }}"
 
       - name: Install actionlint
         run: |