[ruby/rubygems] Default RGV to "." and add switch_rubygems to bin/rspec

With spec/ at the repo root, rspec auto-adds lib/ to $LOAD_PATH,
which now points to the repo's rubygems lib instead of bundler's lib.
This causes a conflict between system and local rubygems. Adding
switch_rubygems ensures a clean local rubygems load from the start.

Also default RGV to "." so no environment variables are needed to
run bin/rspec or bin/parallel_rspec.

https://github.com/ruby/rubygems/commit/b4774e3aab

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

spec/bin/rspec

@@ -1,6 +1,7 @@
 #!/usr/bin/env ruby
 # frozen_string_literal: true
 
+require_relative "../bundler/support/switch_rubygems"
 require_relative "../bundler/support/rubygems_ext"
 
 Spec::Rubygems.gem_load("rspec-core", "rspec")

spec/bundler/support/switch_rubygems.rb

@@ -1,4 +1,5 @@
 # frozen_string_literal: true
 
 require_relative "rubygems_version_manager"
+ENV["RGV"] ||= "."
 RubygemsVersionManager.new(ENV["RGV"]).switch