returned back dotnet-version: 3.1.100

Author: Anton Vorontsov

.github/workflows/github-action.yml

@@ -18,6 +18,8 @@ jobs:
     - uses: actions/checkout@v2
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: 3.1.100
     - name: Build
       run: dotnet build --configuration Release
     - name: Run tests