Update checkout action to current version.

checkout v2 uses node.js v12 which is has been recently deprecated by
github so switch to using the one on the main branch.

.github/workflows/linux_test_asan.yml

@@ -18,6 +18,6 @@ jobs:
       CTEST_OUTPUT_ON_FAILURE: 1
     name: ${{ matrix.compiler }} - ${{ matrix.os }}
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@main
     - name: Run CI script
       run: ./scripts/test