DotNetCoreBuildSettings MSBuildSettings AddFileLogger() does not produce msbuild.log

With `dotnet --version` == `2.0.0`

```csharp
Task("Build")
.IsDependentOn("Restore-NuGet-Packages")
.Does(() =>
{
DotNetCoreBuild("./TheSolution.sln", new DotNetCoreBuildSettings
{
Configuration = configuration,
MSBuildSettings = new DotNetCoreMSBuildSettings
{
TreatAllWarningsAs = MSBuildTreatAllWarningsAs.Error,
Verbosity = DotNetCoreVerbosity.Minimal
}.AddFileLogger()
});
});
```

does not produce the expected `msbuild.log` file.

Running `dotnet build /fl` successfully creates the file, though.

Reproduce the issue with dotnet 2.0.0 using the shown DotNetCoreBuild call, especially DotNetCoreBuildSettings.MSBuildSettings and AddFileLogger(). Compare its output with running dotnet build /fl; done means the Cake invocation produces the expected msbuild.log file.

Written by the indexing model from the issue text.