CI: Use latest version of checkout action

Older versions trigger deprecation warnings.

Author: tarleb

.github/workflows/ci.yaml

@@ -37,7 +37,7 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
 
       - name: Install dependencies
         run: apk add make