Update dotnet-core-desktop.yml

Maybe this will fix it?

Author: dustinsoftware

.github/workflows/dotnet-core-desktop.yml

@@ -39,7 +39,7 @@ jobs:
     - name: Install .NET Core
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: 3.1.401
+        dotnet-version: 3.1.x
 
     - name: Use Node.js 12
       uses: actions/setup-node@v1