Update checkout action

Author: codingben

.github/workflows/build.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
       - name: Setup .NET Core
         uses: actions/setup-dotnet@v1
         with: