Update dotnet.yml

add pre-release info to action

Author: timschmitz86

.github/workflows/dotnet.yml

@@ -17,6 +17,7 @@ jobs:
       uses: actions/setup-dotnet@v1
       with:
         dotnet-version: 6.0.x
+        include-prerelease: true
     - name: Restore dependencies
       run: dotnet restore
     - name: Build