(GH-520) Add random ApiKey

To set the X-NuGet-ApiKey header when pushing to sources like Azure
Artifacts.

Cake.Recipe/Content/packages.cake

@@ -279,6 +279,10 @@ public void PushNuGetPackages(ICakeContext context, bool isRelease, List<Package
  Password = nugetSource.Credentials.Password
  };
 
+ // This is required so that the X-NuGet-ApiKey is set, which is required when
+ // pushing to some sources, for example Azure Artifacts.
+ nugetPushSettings.ApiKey = "RandomToken";
+
  context.Information("Adding NuGet source with user/pass...");
  context.NuGetAddSource(isRelease ? string.Format("ReleaseSource_{0}", nugetSource.Name) : string.Format("PreReleaseSource_{0}", nugetSource.Name), nugetSource.PushUrl, nugetSourceSettings);
  canPushToNuGetSource = true;