Use v2 of checkout action

.github/workflows/codeql.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2
       with:
         # Must fetch at least the immediate parents so that if this is
         # a pull request then we can checkout the head of the pull request.