CI: use v3 of checkout workflow.

Author: mcserep

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Set up Git repository
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
       - if: ${{ matrix.engine == 'pdflatex' }}
         name: Compile LaTeX document
         uses: xu-cheng/latex-action@v2