[rubygems/rubygems] Fix `simulate_windows` helper

It should also set the proper class variable so that `Gem.win_platform?`
returns true.

https://github.com/rubygems/rubygems/commit/0fbf6904d1

spec/bundler/support/hax.rb

@@ -14,6 +14,10 @@ module Gem
     @default_specifications_dir = nil
   end
 
+  if ENV["BUNDLER_SPEC_WINDOWS"]
+    @@win_platform = true # rubocop:disable Sryle/ClassVars
+  end
+
   if ENV["BUNDLER_SPEC_PLATFORM"]
     class Platform
       @local = new(ENV["BUNDLER_SPEC_PLATFORM"])

spec/bundler/support/helpers.rb

@@ -446,11 +446,15 @@ module Spec
     end
 
     def simulate_windows(platform = mswin)
+      old = ENV["BUNDLER_SPEC_WINDOWS"]
+      ENV["BUNDLER_SPEC_WINDOWS"] = "true"
       simulate_platform platform do
         simulate_bundler_version_when_missing_prerelease_default_gem_activation do
           yield
         end
       end
+    ensure
+      ENV["BUNDLER_SPEC_WINDOWS"] = old
     end
 
     def simulate_bundler_version_when_missing_prerelease_default_gem_activation