(build) Add override for GitHub Token Variable

The environment varibles used in this project differ slightly from the
default, so adding override for the token variable.

Author: gep13

recipe.cake

@@ -1,7 +1,8 @@
 #load nuget:https://www.myget.org/F/cake-contrib/api/v2?package=Cake.Recipe&version=2.0.0-unstable0023&prerelease
 
 Environment.SetVariableNames(githubUserNameVariable: "GITTOOLS_GITHUB_USERNAME",
-                            githubPasswordVariable: "GITTOOLS_GITHUB_PASSWORD");
+                            githubPasswordVariable: "GITTOOLS_GITHUB_PASSWORD",
+                            githubTokenVariable: "GITTOOLS_GITHUB_TOKEN");
 
 BuildParameters.SetParameters(context: Context,
                             buildSystem: BuildSystem,