[rubygems/rubygems] handle removing `BUNDLE_`, since using start_with? would still include that

https://github.com/rubygems/rubygems/commit/235d9b38d8

lib/bundler/settings.rb

@@ -90,8 +90,10 @@ module Bundler
     def initialize(root = nil)
       @root            = root
       @local_config    = load_config(local_config_file)
+
       @env_config      = ENV.to_h
-      @env_config.select! {|key, _value| key.start_with?("BUNDLE_") }
+      @env_config.select! {|key, _value| key.start_with?("BUNDLE_")}
+      @env_config.delete("BUNDLE_")
 
       @global_config   = load_config(global_config_file)
       @temporary       = {}