Add UntrustedLocation check

[JanKrivanek]

Fixes dotnet/sdk#45342

Context

Building (or just executing targets) under the Downloads folder is not recomended, as uninteded injection of unrelated msbuild logic can happen.

This checks emits error in such case.

Testing

Manual testing

PS C:\Users\jankrivanek\Downloads\mytest\console> C:\src\msbuild-2\artifacts\bin\bootstrap\net472\MSBuild\Current\Bin\MSBuild.exe -check -v:m -restore
MSBuild version 17.14.0-dev-25064-01+a273f56ac for .NET Framework
  Determining projects to restore...
  Restored C:\Users\jankrivanek\Downloads\mytest\console\console.csproj (in 205 ms).
MSBUILD : error BC0301: Location: 'C:\Users\jankrivanek\Downloads\mytest\console' cannot be fully trusted, place your proje
cts outside of that folder (Project: console.csproj).
  console -> C:\Users\jankrivanek\Downloads\mytest\console\bin\Debug\net9.0\console.dll

[JanKrivanek]

Thanks @YuliiaKovalova for helping to improve the Downloads location obtaining logic!