platform_family? helper is part of recipe DSL, not node object
The Omnibus framework already 'busts' GEM_HOME and GEM_PATH

https://github.com/opscode/omnibus-ruby/blob/master/lib/omnibus/builder.rb#L93-L97
https://github.com/opscode/omnibus-ruby/blob/master/lib/omnibus/builder.rb#L147-L149