Revert "Simplify bin_path_spec.rb guard"

This reverts commit a56d742.

I was checking the CI result of the wrong revision. It actually worked
fine https://ci.appveyor.com/project/ruby/ruby/builds/27866303. Never
mind...

spec/ruby/library/rubygems/gem/bin_path_spec.rb

@@ -11,7 +11,7 @@
     ENV['BUNDLE_GEMFILE'] = @bundle_gemfile
   end
 
-  platform_is_not :windows do
+  guard_not -> { platform_is :windows } do
     it "finds executables of default gems, which are the only files shipped for default gems" do
       # For instance, Gem.bin_path("bundler", "bundle") is used by rails new