Add Rugged search path to Gitaly config

By default, Rugged searches inside /etc/gitconfig instead of /opt/gitlab/embedded/etc/gitconfig for system-wide options unless the Rugged::Settings['search_path_system'] is set.

This means that important options such as core.fsyncObjectFiles are not propagated to gitaly-ruby, which can result in data loss after servers are rebooted. We worked around this problem by propagating the core options in the user's .gitconfig file in !3648 (merged), but we shouldn't need to do this.

The command-line git doesn't have this issue because it uses the prefix compile-time option, but this isn't available in Rugged.