Fix ToolPath for paket push

Author: Tarmil

WebSharper.Fake.fsx

@@ -437,6 +437,10 @@ let MakeTargets (args: Args) =
             Trace.tracefn "[NUGET] Publishing to %s" nugetPublishUrl
             Paket.push <| fun p ->
                 { p with
+                    ToolPath =
+                        if Environment.isWindows
+                        then @".paket\paket.exe"
+                        else @".paket/paket"
                     PublishUrl = nugetPublishUrl
                     WorkingDir = "build"
                 }