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 573ffad.

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 573ffad 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.

Author: Edouard-chin

bundler/lib/bundler/endpoint_specification.rb

@@ -60,6 +60,28 @@ def load_paths
       end
     end
 
+    # needed for binstubs
+    def executables
+      if @remote_specification
+        @remote_specification.executables
+      elsif _local_specification
+        _local_specification.executables
+      else
+        super
+      end
+    end
+
+    # needed for bundle clean
+    def bindir
+      if @remote_specification
+        @remote_specification.bindir
+      elsif _local_specification
+        _local_specification.bindir
+      else
+        super
+      end
+    end
+
     # needed for post_install_messages during install
     def post_install_message
       if @remote_specification

bundler/spec/commands/install_spec.rb

@@ -1885,6 +1885,25 @@ def run
     expect(Dir.glob(vendored_gems("bin/*"))).to eq(expected_executables)
   end
 
+  it "prevents removing binstubs when BUNDLE_CLEAN is set" do
+    build_repo4 do
+      build_gem "kamal", "4.0.6" do |s|
+        s.executables = ["kamal"]
+      end
+    end
+
+    gemfile = <<~G
+      source "https://gem.repo4"
+      gem "kamal"
+    G
+
+    install_gemfile(gemfile, env: { "BUNDLE_CLEAN" => "true", "BUNDLE_PATH" => "vendor/bundle" })
+
+    expected_executables = [vendored_gems("bin/kamal").to_s]
+    expected_executables << vendored_gems("bin/kamal.bat").to_s if Gem.win_platform?
+    expect(Dir.glob(vendored_gems("bin/*"))).to eq(expected_executables)
+  end
+
   it "preserves lockfile versions conservatively" do
     build_repo4 do
       build_gem "mypsych", "4.0.6" do |s|