update checkout action version (v2 to v4)

Author: fangohr

.github/workflows/binder-nbval.yml

@@ -11,7 +11,7 @@ jobs:
   repo2docker-build-and-test:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
 
     - name: Build Docker Container with repo2docker
       uses: jupyterhub/repo2docker-action@master