"ToolLocationHelper.GetPathToBuildToolsFile" returns null if running under net core 2.1

### Steps to reproduce

1) Create a new dotnet console app in Visual Studio
2) Add nuget package Microsoft.Build.Utilities.Core
3) Try to find where msbuild 15.x is located:

`var path=ToolLocationHelper.GetPathToBuildToolsFile("msbuild.exe", buildversion);`

**Result is NULL under NetCore**

3) Create a new .NET 4.7.1 console app in Visual Studio
4) Add nuget package Microsoft.Build.Utilities.Core
5) Repeat under .net frameword 4.7.1 the search again:

`var path=ToolLocationHelper.GetPathToBuildToolsFile("msbuild.exe", buildversion);`

Result is MSBUILD PATH under Visual Studio 2017 as expected.

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Since ToolLocationHelper can be used via Nuget in both circunstances... I think there is a problem with dotnet, not been able to obtain msbuild location.