Update actions/checkout action to v6

Author: whistlerbot

.github/workflows/build.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Build
         run: |
           docker run --rm -v $(pwd):/app -w /app rust:alpine sh -c "