Fix set not being found when running specs on dev rubies

Author: deivid-rodriguez

bundler/spec/runtime/inline_spec.rb

@@ -451,6 +451,9 @@ def confirm(msg, newline = nil)
 
     realworld_system_gems "timeout uri" # this spec uses net/http which requires these default gems
 
+    # on prerelease rubies, a required_rubygems_version constraint is added by RubyGems to the resolution, causing Molinillo to load the `set` gem
+    realworld_system_gems "set --version 1.0.3" if Gem.ruby_version.prerelease?
+
     script <<-RUBY, :dir => tmp("path_without_gemfile"), :env => { "BUNDLER_GEM_DEFAULT_DIR" => system_gem_path.to_s }
       require "bundler/inline"