Use preinstalled dotnet to build

Author: arthurkehrwald

.github/workflows/build.yml

@@ -23,9 +23,6 @@ jobs:
             rid: linux-x64
     steps:
       - uses: actions/checkout@v2
-      - uses: actions/setup-dotnet@v1
-        with:
-          dotnet-version: '3.1.x'
       - name: Build
         run: |
           dotnet build -c Release -r ${{ matrix.rid }}