Exercise the malicious extensions check on Windows

The example built a gem on disk, and util_build_gem writes every entry of
spec.files, which includes spec.extensions. On Windows the extension name
embedding a newline cannot become a file name, so the example was skipped
and the check went uncovered. Build the installer from the spec in memory
with Installer.for_spec instead, matching the sibling non_string checks, so
verify_spec still runs before the spec is eval'd without the newline ever
reaching the disk.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: hsbt

test/rubygems/test_gem_installer.rb

@@ -1901,22 +1901,14 @@ def spec.validate(*args); end
   end
 
   def test_pre_install_checks_malicious_extensions_before_eval
-    pend "mswin environment disallow to create file contained the carriage return code." if Gem.win_platform?
-
     spec = util_spec "malicious", "1"
-    def spec.full_name # so the spec is buildable
-      "malicious-1"
-    end
-
     def spec.validate(*args); end
     spec.extensions = ["malicious\n``"]
 
-    util_build_gem spec
-
-    gem = File.join(@gemhome, "cache", spec.file_name)
+    installer = Gem::Installer.for_spec spec
+    installer.gem_home = @gemhome
 
     use_ui @ui do
-      installer = Gem::Installer.at gem
       e = assert_raise Gem::InstallError do
         installer.pre_install_checks
       end