Azure Worker Role + PostSharp + TFS Online

Question

I am using PostSharp 3 Ultimate in my Azure Worker role project. I am using tfs.visualstudio.com for the source control.

Now, PostSharp doesn't like NuGet Package Restore. It requires us to restore all nuget packages before we build the project on the build server -- http://doc.postsharp.net/##nuget-restore

So, I followed this link (http://docs.nuget.org/docs/reference/package-restore-with-team-build) and created a .proj file that first restores all NuGet packages and then build the solution on the build server. This approach works beautifully for my ASP .Net MVC project that I deploy as an Azure Web Site. But when I try to build my Azure Worker project the same way, I get the following error:

Exception Message: Please specify a Visual Studio Solution (.sln) to build. (type BuildFromSolutionException)
Exception Stack Trace:    at System.Activities.Statements.Throw.Execute(CodeActivityContext context)
at System.Activities.CodeActivity.InternalExecute(ActivityInstance instance, ActivityExecutor executor, BookmarkManager bookmarkManager)
at System.Activities.Runtime.ActivityExecutor.ExecuteActivityWorkItem.ExecuteBody(ActivityExecutor executor, BookmarkManager bookmarkManager, Location resultLocation)

Here is how my solution is structured:

+ Solution
|-+ packages
|-+ Tools
| |-+ NuGet
|   |-- NuGet.exe
|-+ Workers (Cloud Project)
|-+ Workers.QueueProcessor (Worker Role)
|-- Workers.Build.proj
|-- Workers.sln

Here is what is in the new Workers.Build.proj file I checked in to the TFS:

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="4.0"
    DefaultTargets="Build" 
    xmlns="http://schemas.microsoft.com/developer/msbuild/2003">

    <PropertyGroup>
        <OutDir>$(MSBuildThisFileDirectory)bin</OutDir>
        <Configuration>Release</Configuration>
        <ProjectProperties>
            OutDir=$(OutDir);
            Configuration=$(Configuration);
        </ProjectProperties>
    </PropertyGroup>

    <ItemGroup>
        <Solution Include="$(MSBuildThisFileDirectory)Workers.sln" />
    </ItemGroup>

    <Target Name="RestorePackages">
        <Exec Command="$(MSBuildThisFileDirectory)Tools\NuGet\NuGet.exe restore $(MSBuildThisFileDirectory)Workers.sln" />
    </Target>

    <Target Name="Clean">
        <MSBuild Targets="Clean"
            Projects="@(Solution)"
            Properties="$(ProjectProperties)" />
    </Target>

    <Target Name="Build" DependsOnTargets="RestorePackages">
        <MSBuild Targets="Build"
            Projects="@(Solution)"
            Properties="$(ProjectProperties)" />
    </Target>

    <Target Name="Rebuild" DependsOnTargets="RestorePackages">
        <MSBuild Targets="Rebuild"
            Projects="@(Solution)"
            Properties="$(ProjectProperties)" />
    </Target>
</Project>