.github: update actions/checkout to v4

Author: benma

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-22.04
     steps:
       - name: Clone the repo
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
         with:
           submodules: recursive
       - name: Pull CI container image