Fix plain build when only latest SDK is installed

[vzarytovskii]

If only latest sdk (8p6 for example) is installed, due to lack of rollforward, it fails to run fslex and fsyacc on plain build with:

 App: D:\a\_work\1\s\artifacts\bin\fslex\Debug\net7.0\fslex.dll
  Architecture: x64
  Framework: 'Microsoft.NETCore.App', version '7.0.0' (x64)
  .NET location: D:\a\_work\_tool\dotnet\
  
  The following frameworks were found:
    8.0.0-preview.6.23329.7 at [D:\a\_work\_tool\dotnet\shared\Microsoft.NETCore.App]
  
  Learn more:
  https://aka.ms/dotnet/app-launch-failed
  
  To install missing framework, download:
  https://aka.ms/dotnet-core-applaunch?framework=Microsoft.NETCore.App&framework_version=7.0.0&arch=x64&rid=win10-x64
D:\a\_work\1\s\buildtools\buildtools.targets(30,5): error MSB3073: The command ""D:\a\_work\_tool\dotnet\dotnet.exe" "D:\a\_work\1\s\artifacts//bin/fslex/Debug/net7.0/fslex.dll" -o "D:\a\_work\1\s\artifacts/obj/FSharp.Compiler.Service/Debug/netstandard2.0\illex.fs" --module FSharp.Compiler.AbstractIL.AsciiLexer --internal --open Internal.Utilities.Text.Lexing --open FSharp.Compiler.AbstractIL.AsciiParser --unicode --lexlib Internal.Utilities.Text.Lexing AbstractIL\illex.fsl" exited with code -2147450730. [D:\a\_work\1\s\src\Compiler\FSharp.Compiler.Service.fsproj::TargetFramework=netstandard2.0]

This fixes it, it will use latest runtime to run it.