MDEV-34617 galera.galera_ist_mariabackup_verify_ca fails on FreeBSD

[grooverdan]

trafficstars

• [x] The Jira issue number for this PR is: MDEV-34617

Description

Was failing because innodb-log-file-buffering is a Linux/Windows only variable.

This was introduced in MDEV-33787 to enforce O_DIRECT on Linux.

Release Notes

low importance

How can this PR be tested?

FreeBSD-14 mtr

Basing the PR against the correct MariaDB version

• [ ] This is a new feature or a refactoring, and the PR is based against the latest MariaDB development branch.
• [X] This is a bug fix, and the PR is based against the earliest maintained branch in which the bug can be reproduced.

PR quality check

• [X] I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• [X] For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.