rubyserver: Pass on BUNDLE_USER_CONFIG if set
On my local development environment I've set BUNDLE_USER_CONFIG to point to ~/.config/bundle. This file contains:
BUNDLE_PATH: "vendor/bundle"
So because BUNDLE_PATH is not set directly in the environment, it was not picked up by Gitaly when it spawns gitaly-ruby.
With this change BUNDLE_USER_CONFIG is also propagated when bundle-exec(1)'ing the Ruby sidecar, so it will be able to find the gems.