Update actions/checkout version to v6

Author: linkdata

.github/workflows/self-test.yml

@@ -30,7 +30,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
       - name: Check out action source
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           path: action-src