ILLink task should require setting ToolPath and ToolExe values

Currently if ToolPath and ToolExe are not specified the task implements a default which relies on `DOTNET_HOST_PATH` variable. Which is not very reliable as that's basically an internal implementation detail of the .NET SDK. For example VS test host doesn't set this variable even for tests. Let alone if something tries to run the task on its own.

We don't support using the task on its own - we only support it through SDK which always sets ToolPath/ToolExe. We should require all usages to do that to make the task more robust.