Fix test-bundler failures when XDG_CONFIG_HOME is not writable

https://github.com/ruby/actions/runs/2175399707?check_suite_focus=true
```
Errno::EACCES: Permission denied @ dir_s_mkdir - /home/runner/.config/irb
```

spec/bundler/support/path.rb

@@ -193,6 +193,10 @@ module Spec
       root.join("lib")
     end
 
+    def xdg_config_home
+      home(".config")
+    end
+
     def global_plugin_gem(*args)
       home ".bundle", "plugin", "gems", *args
     end

spec/bundler/support/rubygems_ext.rb

@@ -33,6 +33,7 @@ module Spec
 
       ENV["HOME"] = Path.home.to_s
       ENV["TMPDIR"] = Path.tmpdir.to_s
+      ENV["XDG_CONFIG_HOME"] = Path.xdg_config_home.to_s
 
       require "rubygems/user_interaction"
       Gem::DefaultUserInteraction.ui = Gem::SilentUI.new