GitHub actions/checkout@v1 --> v2 (#4680)

https://github.com/actions/checkout/releases

.github/workflows/main.yml

@@ -39,7 +39,7 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2
     - name: Initialize submodules
       run: git submodule update --recursive --init