Set TZ environment in Rails ENV to avoid excessive stat calls

We should consider setting the TZ environment variable to :/etc/localtime as Gitaly has done in !1711 (merged):

https://blog.packagecloud.io/eng/2017/02/21/set-environment-variable-save-thousands-of-system-calls/

We've done this on our system (https://gitlab.com/gitlab-cookbooks/gitlab-server/merge_requests/143) to address https://gitlab.com/gitlab-com/infrastructure/issues/4250, but this seems like we really should have set in GitLab by default.