Fix: checkout repo before external link checking

We need this so that `requirements-linkchecker.txt` is available.

.github/workflows/checklinks.yml

@@ -10,6 +10,7 @@ jobs:
     name: Check links
     runs-on: ubuntu-latest
     steps:
+      - uses: actions/checkout@v2
       - name: Install requirements
         run: pip3 install -r requirements-linkchecker.txt
       - name: Run linkchecker