Update actions/checkout.

Author: fukamachi

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ jobs:
         lisp: [sbcl]
 
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v4
       - name: Run tests
         env:
           LISP: ${{ matrix.lisp }}