ci: Update actions/checkout to v4

This removes the following warning:

    Node.js 16 actions are deprecated. Please update the following
    actions to use Node.js 20: actions/checkout@v3. For more information
    see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

.github/workflows/build.yaml

@@ -21,7 +21,7 @@ jobs:
 
  steps:
  - name: Checkout code including full history and submodules
- uses: actions/checkout@v3
+ uses: actions/checkout@v4
  with:
  submodules: true
  fetch-depth: 0