How to get TFS2015 Build (Build.vnext) and NuGet package restore to use custom package sources

Question

I'm trying to get our TFS2015 Build (TFSBuild vnext) going along with VS Team Services.

Thus far most blogs and documentation has been helpful, except for when trying to build a project that uses custom package sources for NuGet automatic package restore.

I've successfully deployed an a Team Services Build Agent (the new mechanism for builds) and all seemed to go well until the build task to Restore NuGet packages failed.

The failure was due to custom/private package source not being picked up from the installed VS configuration, so those packages could not be found. The traditional packages from the default NuGet package feed did restore without issue.

How do I specify the additional package sources for NuGet to use when using an agent?

EDIT: vcastro asked about the NuGet Installer build step and defining paths in the configuration of that build step. The above is using the Visual Studio Build step and the integrated option to have NuGet restore packages. Refer this image for reference: 

Answer 1

Try adding a NuGet Installer build step before the Visual Studio Build step into your configuration of build to store all your NuGet Packages.

Now pass the location of the private repository that you created as an argument to nuget.exe :

-source "https://www.nuget.org/api/v2/;http://mynugetserver"