[rubygems/rubygems] Mark to exclude examples at ruby/ruby repository

`make test-bundler` on ruby/ruby don't have installed gemspec.
So, `require "psych"` didn't activate psych as default gems on ruby repo.

https://github.com/rubygems/rubygems/commit/f8ca514cbb

spec/bundler/runtime/inline_spec.rb

@@ -656,7 +656,7 @@ RSpec.describe "bundler/inline#gemfile" do
     expect(out).to include("after: [\"Test_Variable\"]")
   end
 
-  it "does not load specified version of psych and stringio" do
+  it "does not load specified version of psych and stringio", :ruby_repo do
     build_repo4 do
       build_gem "psych", "999"
       build_gem "stringio", "999"