[rubygems/rubygems] Unset ENV set by official Ruby docker images

So that it does not interfere with our specs.

https://github.com/rubygems/rubygems/commit/1029714e21

spec/bundler/spec_helper.rb

@@ -74,6 +74,8 @@ RSpec.configure do |config|
     ENV["BUNDLER_SPEC_RUN"] = "true"
     ENV["BUNDLER_NO_OLD_RUBYGEMS_WARNING"] = "true"
     ENV["BUNDLE_USER_CONFIG"] = ENV["BUNDLE_USER_CACHE"] = ENV["BUNDLE_USER_PLUGIN"] = nil
+    ENV["BUNDLE_APP_CONFIG"] = nil
+    ENV["BUNDLE_SILENCE_ROOT_WARNING"] = nil
     ENV["RUBYGEMS_GEMDEPS"] = nil
     ENV["XDG_CONFIG_HOME"] = nil
     ENV["GEMRC"] = nil