[rubygems/rubygems] Disable parallel installation by default on Windows

Since we enabled parallel installation by default, we've fixed all the
issues related to it that have come up, except for a Windows issue that
we haven't yet figured out. This issue is hit by our specs on a daily
basis and there's no reason to believe that it won't be hit by end users
in a similar way.

So, both to stop the testing flakyness and to prevent regressions in the
default behavior on Windows, I'd rather leave the default as it was
before on Windows for now.

https://github.com/rubygems/rubygems/commit/5d8ab57131

lib/bundler/installer.rb

@@ -202,7 +202,14 @@ module Bundler
         return jobs
       end
 
-      Bundler.settings[:jobs] || processor_count
+      if jobs = Bundler.settings[:jobs]
+        return jobs
+      end
+
+      # Parallelization has some issues on Windows, so it's not yet the default
+      return 1 if Gem.win_platform?
+
+      processor_count
     end
 
     def processor_count