roslyn: MSBuildWorkspace.OpenProjectAsync throws InvalidOperationException when invoked from custom MsBuild task

I havent found anywhere you wouldn’t support this scenario, and even though its a bit on the exotic side, I’ll report it in case you want to dig into it.

Steps to Reproduce:

1. Create a custom MsBuild task in a standard C# library, e.g. similar to this.
2. In Execute(), add the code below.
3. Invoke it from a project (standard Console Application will do fine).

In Task.Execute()

var ws = MSBuildWorkspace.Create();
var project = ws.OpenProjectAsync("C:\path\to\some\standard\project.csproj").Result;

Expected Behavior: A new Project instance.

Actual Behavior: An (aggregated) InvalidOperationException:

System.AggregateException was unhandled by user code
  HResult=-2146233088
  Message=One or more errors occurred.
  Source=mscorlib
  StackTrace:
       at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
       at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)
       at System.Threading.Tasks.Task`1.get_Result()
       at App1.ProjectWeaver..ctor(String projectFilePath) in C:\Users\Per\proj\RoslynWorkspaceLabs\App1\ProjectWeaver.cs:line 89
       at App1.ProjectFileWeaver.Execute() in C:\Users\Per\proj\RoslynWorkspaceLabs\App1\ProjectWeaver.cs:line 28
       at WeaverTasks.WeaverProtoTask.Execute() in C:\Users\Per\proj\RoslynWorkspaceLabs\WeaverTasks\WeaverProtoTask.cs:line 74
       at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute()
       at Microsoft.Build.BackEnd.TaskBuilder.<ExecuteInstantiatedTask>d__26.MoveNext()
  InnerException: 
       HResult=-2146233079
       Message=Cannot open project 'C:\Users\Per\proj\RoslynWorkspaceLabs\ConsoleApplication1\ConsoleApplication1.csproj' because the language 'C#' is not supported.
       Source=Microsoft.CodeAnalysis.Workspaces.Desktop
       StackTrace:
            at Microsoft.CodeAnalysis.MSBuild.MSBuildProjectLoader.ReportFailure(ReportMode mode, String message, Func`2 createException)
            at Microsoft.CodeAnalysis.MSBuild.MSBuildProjectLoader.TryGetLoaderFromProjectPath(String projectFilePath, ReportMode mode, IProjectFileLoader& loader)
            at Microsoft.CodeAnalysis.MSBuild.MSBuildProjectLoader.<LoadProjectInfoAsync>d__20.MoveNext()
         --- End of stack trace from previous location where exception was thrown ---
            at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
            at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
            at Microsoft.CodeAnalysis.MSBuild.MSBuildWorkspace.<OpenProjectAsync>d__19.MoveNext()
       InnerException: 

More: If I do the same in a separate process / standard application, it works like a charm.