[Setup] Dont install bundler when there is no default specs dir

Author: segiddins

lib/rubygems/commands/setup_command.rb

@@ -352,6 +352,8 @@ def fake_spec.full_gem_path
   def install_default_bundler_gem
     return unless Gem::USE_BUNDLER_FOR_GEMDEPS
 
+    return unless File.directory?(Gem::Specification.default_specifications_dir)
+
     bundler_spec = Gem::Specification.load("bundler/bundler.gemspec")
     bundler_spec.files = Dir["bundler/{*.md,{lib,exe,man}/**/*}"]
     bundler_spec.executables -= %w[bundler bundle_ruby]