Add test that load_env_plugins is called

bundler/spec/commands/install_spec.rb

@@ -432,6 +432,17 @@
         gem 'foo'
       G
     end
+
+    it "loads env plugins" do
+      plugin_msg = "hello from an env plugin!"
+      create_file "plugins/rubygems_plugin.rb", "puts '#{plugin_msg}'"
+      gemfile = <<-G
+        source "#{file_uri_for(gem_repo1)}"
+        gem "rack"
+      G
+      out = install_gemfile gemfile, :env => { "RUBYLIB" => bundled_app("plugins/").to_s }
+      expect(out).to include(plugin_msg)
+    end
   end
 
   describe "Ruby version in Gemfile.lock" do