Update actions/checkout to v4

Fix following warning message in CI:

```
Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: actions/checkout@v2. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.
```

Because the CI will eventually stop working.

Author: Watson1978

.github/workflows/build.yml

@@ -14,7 +14,7 @@ jobs:
         include:
           - { os: windows-2022 , ruby: mswin }
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Set up Ruby ${{ matrix.ruby }}
         uses: ruby/setup-ruby@v1
         with:
@@ -37,7 +37,7 @@ jobs:
       matrix:
         ruby: [ 'debug' ]
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Set up Ruby ${{ matrix.ruby }}
         uses: ruby/setup-ruby@v1
         with: