Revert "(build) use a different env var name for GITHUB_TOKEN to avoi…

…d collision"

This reverts commit 7aed1f8.

build/utils/credentials.cake

@@ -36,7 +36,7 @@ public class GitHubCredentials
     {
         return new GitHubCredentials(
             context.EnvironmentVariable("GITHUB_USERNAME"),
-            context.EnvironmentVariable("RELEASE_GITHUB_TOKEN"));
+            context.EnvironmentVariable("GITHUB_TOKEN"));
     }
 }