[rubygems/rubygems] Add missing Windows job

Specs that use extension gems were failing in the new job but I noticed
that they were using very non standard `extconf.rb` files.

The hack being removed here was added just to make specs pass when run
in ruby-core but it seems the underlying issue has been fixed now, and
it's causing issues with Ruby 3.3 and Windows, so necessary so I'm
removing it and moving on.

rubygems/rubygems@5b78275f0e

Author: deivid-rodriguez

spec/bundler/install/gemfile/git_spec.rb

@@ -1279,7 +1279,6 @@
         s.extensions = ["ext/extconf.rb"]
         s.write "ext/extconf.rb", <<-RUBY
           require "mkmf"
-          $extout = "$(topdir)/" + RbConfig::CONFIG["EXTOUT"]
           create_makefile("foo")
         RUBY
         s.write "ext/foo.c", "void Init_foo() {}"

spec/bundler/install/gems/native_extensions_spec.rb

@@ -7,7 +7,6 @@
         s.extensions = ["ext/extconf.rb"]
         s.write "ext/extconf.rb", <<-E
           require "mkmf"
-          $extout = "$(topdir)/" + RbConfig::CONFIG["EXTOUT"]
           name = "c_extension_bundle"
           dir_config(name)
           raise "OMG" unless with_config("c_extension") == "hello"
@@ -52,7 +51,6 @@
       s.extensions = ["ext/extconf.rb"]
       s.write "ext/extconf.rb", <<-E
         require "mkmf"
-        $extout = "$(topdir)/" + RbConfig::CONFIG["EXTOUT"]
         name = "c_extension_bundle"
         dir_config(name)
         raise "OMG" unless with_config("c_extension") == "hello"
@@ -97,7 +95,6 @@
           s.extensions = ["ext/extconf.rb"]
           s.write "ext/extconf.rb", <<-E
             require "mkmf"
-            $extout = "$(topdir)/" + RbConfig::CONFIG["EXTOUT"]
             name = "c_extension_bundle_#{n}"
             dir_config(name)
             raise "OMG" unless with_config("c_extension_#{n}") == "#{n}"
@@ -150,7 +147,6 @@
       s.extensions = ["ext/extconf.rb"]
       s.write "ext/extconf.rb", <<-E
         require "mkmf"
-        $extout = "$(topdir)/" + RbConfig::CONFIG["EXTOUT"]
         name = "c_extension_bundle"
         dir_config(name)
         raise "OMG" unless with_config("c_extension") == "hello" && with_config("c_extension_bundle-dir") == "hola"

spec/bundler/support/builders.rb

@@ -494,8 +494,6 @@ def add_c_extension
         write "ext/extconf.rb", <<-RUBY
           require "mkmf"
 
-          $extout = "$(topdir)/" + RbConfig::CONFIG["EXTOUT"]
-
           extension_name = "#{name}_c"
           if extra_lib_dir = with_config("ext-lib")
             # add extra libpath if --with-ext-lib is