This makes bundler consistent with all other gems, and makes the default
installation of Bundler in the release package look like any other
bundler installation.

Before (on preview3, for example), Bundler executable is installed at:

lib/ruby/gems/3.3.0+0/gems/bundler-2.5.0.dev/libexec/bundle

Now it's installed in the standard location:

lib/ruby/gems/3.3.0+0/gems/bundler-2.5.0.dev/exe/bundle