Fix git-latest ansible-lint errors

Reference: https://ansible-lint.readthedocs.io/en/latest/default_rules/#git-latest

Our variable naming is not necessarily consistent across roles.
I've tried to follow the naming conventions of each individual role.
All new variables are suffixed with `_version`, but the prefix may be
somewhat different.

roles/matrix-backup-borg/tasks/setup_install.yml

@@ -63,6 +63,7 @@
 - name: Ensure borg repository is present on self-build
   ansible.builtin.git:
     repo: "{{ matrix_backup_borg_docker_repo }}"
+    version: "{{ matrix_backup_borg_docker_repo_version }}"
     dest: "{{ matrix_backup_borg_docker_src_files_path }}"
     force: "yes"
   become: true