IsolatedScriptingContainer does not honour setting Environment or Argv

```
c = IsolatedScriptingContainer.new
c.argv = ['-debug']
c.environment = { 'GEM_PATH' => '/tmp' }
c.runScriptlet("p ARGV; p ENV")
```

the configuration does not show up on ARGV or ENV. it works on 9k but not on 1.7.x since the constructor of IsolatedScriptingContainer does run a scriptlet already and then all changes to ARGV and ENV get ignored.
