Update appveyor: Use .net core 3 preview sdk

Author: dustinsoftware

appveyor.yml

@@ -4,10 +4,20 @@ install:
     - set PATH=%ProgramFiles(x86)%\Microsoft Visual Studio\2017\Community\MSBuild\15.0\Bin\;%PATH%
     - ps: Install-Product node 10
     - npm install --global npm
+    - ps: |
+        Write-Host "Installing .NET Core SDK..." -ForegroundColor Cyan
+        Write-Host "Downloading..."
+        $exePath = "$env:TEMP\dotnet-sdk.exe"
+        (New-Object Net.WebClient).DownloadFile('https://download.visualstudio.microsoft.com/download/pr/31b5b67f-b787-4f73-a728-5ec61f10a4de/be6430bcd9a62f610cd9f12f8cc2c736/dotnet-sdk-3.0.100-preview3-010431-win-x64.exe', $exePath)
+        Write-Host "Installing..."
+        cmd /c start /wait "$exePath" /quiet /norestart
+        del $exePath
+        Write-Host "Installed" -ForegroundColor Green
 build:
     project: build.proj
     verbosity: normal
 test_script:
     - dotnet test --configuration Release --no-build tests/React.Tests/React.Tests.csproj
 artifacts:
     - path: output\*.nupkg
+