Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/build.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-24.04
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Bootstrap
         run: ./init/bootstrap_ubuntu_server.sh localhost field-a
       - name: Cleanup