RUBYOPT is pulled from system env always, even for in-process child

When launching a child "ruby" or "jruby" instance in the same JVM, we are still looking at System.getenv for RUBYOPT, even though we pass a new pseudo-environment along to the pseudo-child. This causes RubyGems update to fail on older versions of JRuby since a RUBYOPT=rubygems env var will cause both versions of RubyGems to get loaded.

This causes JRUBY-5509, which is also tracked in the following RubyGems issue:

http://rubyforge.org/tracker/index.php?func=detail&aid=28924&group_id=126&atid=575

Fix pending.