Manually update .NET Core on appveyor.

appveyor.yml

@@ -6,6 +6,46 @@ max_jobs: 15
 image: Visual Studio 2019 Preview
 configuration: Release
 platform: Any CPU
+install:
+- ps: >-
+    if ($env:APPVEYOR_BUILD_WORKER_IMAGE -eq "Visual Studio 2019 Preview")
+
+    {
+
+    Write-Host "Installing .NET Core SDK..." -ForegroundColor Cyan
+
+    Write-Host "Downloading..."
+
+    $exePath = "$env:TEMP\dotnet-sdk-3.0.100-preview-010184-win-x64.exe"
+
+    (New-Object Net.WebClient).DownloadFile('https://download.visualstudio.microsoft.com/download/pr/a7e73d05-4e75-4543-ac41-fc69d2f617e5/3f65a44f90c9df66664b44e86b0e6d29/dotnet-sdk-3.0.100-preview-010184-win-x64.exe', $exePath)
+
+    Write-Host "Installing..."
+
+    cmd /c start /wait "$exePath" /quiet /norestart
+
+    del $exePath
+
+    Write-Host "Installed" -ForegroundColor Green
+
+
+    Write-Host "Installing .NET Core runtime..." -ForegroundColor Cyan
+
+    Write-Host "Downloading..."
+
+    $exePath = "$env:TEMP\dotnet-runtime-3.0.0-preview-27324-5-win-x64.exe"
+
+    (New-Object Net.WebClient).DownloadFile('https://download.visualstudio.microsoft.com/download/pr/30594a10-9dcf-44c9-88c2-c559da2a0c6e/7cb211623fefc681750632dc6e5b3b36/dotnet-runtime-3.0.0-preview-27324-5-win-x64.exe', $exePath)
+
+    Write-Host "Installing..."
+
+    cmd /c start /wait "$exePath" /quiet /norestart
+
+    del $exePath
+
+    Write-Host "Installed" -ForegroundColor Green
+
+    }
 before_build:
 - ps: nuget restore
 build: