Install .NET 8 on build start

Author: nblumhardt

appveyor.yml

@@ -3,7 +3,10 @@ skip_tags: true
 image: Visual Studio 2022
 test: off
 build_script:
-- ps: ./Build.ps1
+- pwsh: |
+    Invoke-WebRequest "https://dot.net/v1/dotnet-install.ps1" -OutFile "./dotnet-install.ps1"
+    ./dotnet-install.ps1 -JSonFile global.json -Architecture x64 -InstallDir 'C:\Program Files\dotnet'
+    ./Build.ps1
 artifacts:
 - path: artifacts/Serilog.*.nupkg
 skip_commits: