ILLinkTask should not depend on DOTNET_HOST_PATH

It looks like this property is only set when MSBuild is launched by `dotnet build` or `dotnet msbuild`. It won't be present when building from VS, nor desktop msbuild.exe, nor `dotnet msbuild.dll`. I think you should rely on a different mechanism for locating dotnet.exe. Work with folks like @vitek-karas and @nguerrera to find an appropriate convention. I believe ILLink isn't the first tool to hit this.