[ci] Use the latest version of actions/checkout

To avoid the warning about deprecated Node.js 12 actions.  See
https://github.com/ocsigen/ocsigenserver/actions/runs/3672047258.

Author: ilankri

.github/workflows/workflow.yml

@@ -28,7 +28,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
 
       - name: Use OCaml ${{ matrix.ocaml-compiler }}
         uses: ocaml/setup-ocaml@v2