fix: specify path

```
The installation path is insecure. Bundler cannot continue.
/opt/hostedtoolcache/Ruby/3.3.9/x64/lib/ruby/gems/3.3.0/gems is world-writable
(without sticky bit).
Bundler cannot safely replace gems in world-writeable directories due to
potential vulnerabilities.
Please change the permissions of this directory or choose a different install
path.
rake aborted!
```

Author: toshimaru

Rakefile

@@ -73,7 +73,7 @@ def generate_rails_rdoc
       gemfile.gsub!(/"sdoc".*$/, %("sdoc", github: "toshimaru/sdoc", branch: "#{MY_SDOC_BRANCH}"))
       File.write('Gemfile', gemfile)
 
-      sh 'bundle install && bundle update sdoc'
+      sh 'bundle install --path vendor/bundle && bundle update sdoc'
       rm_rf 'doc'
       sh 'bundle exec rake rdoc'
     end