envutil.rb: fix disabling gems in assert_separately
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43436 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
envutil.rb: fix disabling gems in assert_separately
--disable=gems option.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43436 b2dd03c8-39d4-4d8f-98ff-823fe69b080e