Download dotnet install and install net8

Author: wallymathieu

appveyor.yml

@@ -11,7 +11,9 @@ before_build:
 init:
   - git config --global core.autocrlf input
 install:
-  - cmd: winget install Microsoft.DotNet.SDK.8
+  - ps: Invoke-WebRequest 'https://dot.net/v1/dotnet-install.ps1' -OutFile 'dotnet-install.ps1'
+  - ps: ./dotnet-install.ps1 -Version 8.0.100 -InstallDir "C:\Program Files\dotnet"
+  #- cmd: winget install Microsoft.DotNet.SDK.8
   - cmd: git submodule update --init --recursive
 build_script:
   - cmd: dotnet restore ./FSharpPlus.sln