Update CI.yml

Author: GlebChili

.github/workflows/CI.yml

@@ -15,7 +15,7 @@ jobs:
 
     steps:
     - name: Checkout
-      uses: actions/checkout@v2
+      uses: actions/checkout@v2.3.4
 
     - name: Setup .NET Core 3.1 SDK
       uses: actions/setup-dotnet@v1.7.2