unable to locate nuget.exe when building webservice on appharbor

I solved this by changing this line in my NuGet.targets file and setting it to true:

<DownloadNuGetExe Condition=" '$(DownloadNuGetExe)' == '' ">true</DownloadNuGetExe>

But you must restart Visual Studio or reload the solution (see this) for this to take effect.