house-keeping: migrate workflows from GitHub deprecated node16 -> node20

GitHub currently warns with:
Annotations
1 warning
build

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.yml

@@ -15,7 +15,7 @@ jobs:
             target: x86-64
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
         with:
           fetch-depth: 0
 

.github/workflows/formal.yml

@@ -11,7 +11,7 @@ jobs:
       fail-fast: false
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
         with:
           ref: ${{ github.event.pull_request.head.sha }}
           fetch-depth: 0