Simplest way in FAKE to restore packages with a source

Question

I was wondering what is the simplest way to restore packages when there is an alternative source. I tried

Target "RestorePackages" (fun _ ->
!! "./**/packages.config"
    |> Seq.iter (RestorePackage (fun p ->
        { p with Sources = ["http://mysource.com"]}))

)

This works locally but I was wondering if there is a way to call

RestorePackages()

without having to set up nuget configuration? Is there any other (nicer)way ?