Do not require the simple_po_parser in the gettext rake task when in a

What does this MR do?

Only require the po linter library when we are not loading a production environment.

Followup to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14018

cc\ @reprazent

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Without this change we get the error when running bundle exec rake gettext:compile

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/37379