PATH in builds

[patrobinson]

As per #2744 the PATH variable leaks into our jobs and this can impact using rbenv.
When samson is launched with bundle exec puma, it prepends the directory /var/www/samson/releases/1/vendor/bundle/ruby/2.5.0/bin to the PATH.

It appears this issue was introduced in 2bf25bd
Previously we were calling:

Bundler.with_clean_env do
  PTY.spawn(...)
end

This strips the path prepended by bundle exec and hence avoids conflicts with rbenv.

Should we re-introduce Bundler.with_clean_env to strip the PATH?

cc @grosser

[grosser]

kk, fix coming in a week or so ... PR welcome if you know how to fix