[rubygems/rubygems] Make test env resilient to ruby version changes

We use pre-existence of `rake_path` to decide whether we need to
regenerate dummy test gems in `tmp`. When changing rubies, the previous
implementation will believe that the correct `rake_path` exists
and avoids regenerating dummy gems, given an error like the following
when specs are run:

```
(...)
Could not find rubygems-generate_index lib directory in /path/to/rubygems/bundler/tmp/1/gems/base/ruby/3.2.0
  # ./spec/support/builders.rb:253:in `block in update_repo'
  # ./spec/support/helpers.rb:337:in `block in with_gem_path_as'
  # ./spec/support/helpers.rb:351:in `without_env_side_effects'
  # ./spec/support/helpers.rb:332:in `with_gem_path_as'
  # ./spec/support/builders.rb:251:in `update_repo'
  # ./spec/support/builders.rb:228:in `build_repo'
  # ./spec/support/builders.rb:197:in `build_repo4'
  # ./spec/commands/lock_spec.rb:103:in `block (2 levels) in <top (required)>'
(...)
```

To fix this, fix the part of the path that depends on the implementation
and the Ruby version so that we don't give false positives.

https://github.com/rubygems/rubygems/commit/fafacfa210

spec/bundler/support/path.rb

@@ -266,7 +266,7 @@ module Spec
     end
 
     def rake_path
-      Dir["#{base_system_gems}/**/rake*.gem"].first
+      Dir["#{base_system_gems}/#{Bundler.ruby_scope}/**/rake*.gem"].first
     end
 
     private