[ruby/rubygems] Fix Bundler removing executables after creating them

When running a fresh `bundle install` with gems that contains
executables, Bundler will generate binstubs but soon after will remove
them. This is a regression introduced in https://github.com/ruby/rubygems/commit/573ffad3ea4a.

This results in doing `bundle install && bundle exec foo` to raise an
error saying `foo` couldn't be found.

This issue only appears if `BUNDLE_CLEAN` is set.

At the end of the installation process, when Bundler has installed
gems and generated binstubs, it runs the cleanup.

1. It [detects](https://github.com/ruby/rubygems/blob/4f8aa3b40cded3465bb2cd761e9ce7f8673b7fcb/bundler/lib/bundler/runtime.rb#L182) the executable for the current specs
2. Any existing executables not detected is then removed https://github.com/ruby/rubygems/blob/4f8aa3b40cded3465bb2cd761e9ce7f8673b7fcb/bundler/lib/bundler/runtime.rb#L194.

The issue being that 1. now returns an empty array where as it should
return the executables of the gems from the current bundle.

The problem is in https://github.com/ruby/rubygems/commit/573ffad3ea4a where we
removed the `executables` method from the `EndpointSpecification`.
When Bundler reads the lockfile, it creates a `EndpointSpecification`
object for each spec. At this point, the EndpointSpecification doesn't
know about the `executables` of a gem. Once Bundler fetches the
`gemspec` from the remote, it swaps the the "spec" with the real one
and from here knows what executables the gem has.

Reintroduce the `executables` method and the `bindir` in the
EndpointSpecification class. From what I'm seeing, the removal
of those wasn't needed to resolve the issue where Bundler remembers
CLI flags. This is probably an oversight.

https://github.com/ruby/rubygems/commit/b47f6b0247

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