Unified Backups: Detect Omnibus Ruby version in `gitlab-backup-cli`

What does this MR do and why?

This MR fixes using an incorrect Ruby interpreter for gitlab-backup-cli executable. We use a wrapper to identify if the GITLAB_BACKUP_CLI_CONFIG_FILE ENV variable is set, which means we are using Omnibus as the installation type for Gitlab. Otherwise, we use the system Ruby.

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Related to #443660 (closed)