[ruby/rubygems] Retain current bundler version on bundle clean

Previously: In ruby#9218 a reproduction is shared where running `bundle clean` using a binstub (`bin/bundle`) results in bundler removing itself. This results in Ruby falling back to its default bundler version. This behavior seems to be present for as long as there has been a default version of bundler (Ruby 2.6+).

Now: Bundler will explicitly add its current version number to the specs to be preserved. This prevents `bundle clean` from removing the current bundler version.

close ruby/rubygems#9218

ruby/rubygems@e3f0167ae4

Author: schneems

lib/bundler/runtime.rb

@@ -175,6 +175,12 @@ def clean(dry_run = false)
       spec_gemspec_paths   = []
       spec_extension_paths = []
       specs_to_keep = Bundler.rubygems.add_default_gems_to(specs).values
+
+      current_bundler = Bundler.rubygems.find_bundler(Bundler.gem_version)
+      if current_bundler
+        specs_to_keep << current_bundler
+      end
+
       specs_to_keep.each do |spec|
         spec_gem_paths << spec.full_gem_path
         # need to check here in case gems are nested like for the rails git repo