Bump actions/checkout from v2 to v4 (#550)

GitHub Actions workflow prints the following warning.

> The following actions uses node12 which is deprecated and will be forced to run on node16: actions/checkout@v2. For more info: https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/

This PR updates actions/checkout to v4 in order to suppress the warning.

.github/workflows/build.yml

@@ -45,7 +45,7 @@ jobs:
     env:
       RAILS: ${{ matrix.rails }}
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}