Bump checkout action to v6

Author: stuarthayhurst

.github/workflows/test-build-system.yml

@@ -13,7 +13,7 @@ jobs:
   build:
     runs-on: ubuntu-24.04
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: Set up Python 3.14
       uses: actions/setup-python@v6
       with: