default.mspec: ignore unavailable FDs

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58727 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/default.mspec

@@ -49,6 +49,7 @@ module MSpecScript::JobServer
           end
           return cores
         end
+      rescue Errno::EBADF
       ensure
         r&.close
         w&.close