Consistently use tar lookup function

What does this MR do?

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19363 a change in the backup code slipped in. Because we rely on GNU tar in one file, I added a method that checks if a gtar command is available, and then it uses that. Otherwise it falls back to tar.

For consistency, this MR changes that part of the backup script so we also create the tar files with gtar if present. This seems more logical than using tar when backing up, and gtar when restoring.

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

Why was this MR needed?

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
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/47308