Use the built ruby for running fpm

As long as we install fpm and its dependencies outside the build tree, we should be able to use the freshly built ruby. This way we don't need to install system ruby, avoid extra converge phase (for correct Ohai data, hw-cookbooks/fpm-tng#3), and most importantly, don't need to worry strange gem's bindir paths on debianoids (hw-cookbooks/fpm-tng#2).

Problem is to pass the `GEM_HOME` or `--install-dir` to fpm-tng cookbook resources. As some resources are created in the provider, using _.gemrc_ might be the only way even it feels wrong.