Missing Microsoft.Build.Tasks.v4.0.dll on visual studio online tfs build

I am trying to build my project with visual studio online.

I am getting the following error.

C:\a\src\.nuget\nuget.targets (71): The task factory "CodeTaskFactory" could not be loaded from the assembly "C:\Program Files (x86)\MSBuild\12.0\bin\amd64\Microsoft.Build.Tasks.v4.0.dll". Could not load file or assembly 'file:///C:\Program Files (x86)\MSBuild\12.0\bin\amd64\Microsoft.Build.Tasks.v4.0.dll' or one of its dependencies. The system cannot find the file specified.

This is coming from the nuget.targets file in my solution.

  <UsingTask TaskName="SetEnvironmentVariable" TaskFactory="CodeTaskFactory" AssemblyFile="$(MSBuildToolsPath)\Microsoft.Build.Tasks.v4.0.dll">
        <ParameterGroup>
            <EnvKey ParameterType="System.String" Required="true" />
            <EnvValue ParameterType="System.String" Required="true" />
        </ParameterGroup>
        <Task>
            <Using Namespace="System" />
            <Code Type="Fragment" Language="cs">
                <![CDATA[
                try {
                    Environment.SetEnvironmentVariable(EnvKey, EnvValue, System.EnvironmentVariableTarget.Process);
                }
                catch  {
                }
            ]]>
            </Code>
        </Task>
    </UsingTask>

I have updated nuget.exe to latest and have not changed anything in nuget.targets.