Fix the shebang of binaries under embedded/bin on *nix platforms.

Fixes https://github.com/opscode/chef/issues/2379. 

I've taken the last option which is not beautiful but is safe and decreases the code churn in the last minute. Thoughts @lamont-granquist, @schisamo ?
I think we can use Omnibus's logger here. Perhaps try:

```ruby
log.info(log_key) { "Updating shebang for file: '#{bin_file}'" }
```