CI: limit codespell checks to main repo, not forks

.github/workflows/codespell.yml

@@ -10,6 +10,7 @@ permissions:
 jobs:
   codespell:
     runs-on: ubuntu-latest
+    if: ${{ github.repository_owner == 'haproxy' }}
     steps:
     - uses: actions/checkout@v4
     - uses: codespell-project/codespell-problem-matcher@v1