Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/build_tests.yml

@@ -7,7 +7,7 @@ jobs:
   build_tests:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v6
         with:
           ref: ${{ github.head_ref }}
       - name: Setup Python