fix(partitioning): set btrfs default subvolume and restrict @pkg to Arch

roles/partitioning/tasks/btrfs.yml

@@ -43,6 +43,7 @@
       when:
         - system_cfg.features.cis.enabled | bool or item.subvol not in ['var_log_audit']
         - system_cfg.features.swap.enabled | bool or item.subvol != 'swap'
+        - item.os is not defined or os in item.os
       ansible.builtin.command: btrfs su cr /mnt/{{ '@' if item.subvol == 'root' else '@' + item.subvol }}
       args:
         creates: /mnt/{{ '@' if item.subvol == 'root' else '@' + item.subvol }}
@@ -51,12 +52,19 @@
         - { subvol: swap }
         - { subvol: home }
         - { subvol: var }
-        - { subvol: pkg }
+        - { subvol: pkg, os: [archlinux] }
         - { subvol: var_log }
         - { subvol: var_log_audit }
       loop_control:
         label: "{{ item.subvol }}"
 
+    - name: Set default btrfs subvolume to @
+      ansible.builtin.shell: >-
+        btrfs subvolume list /mnt | awk '/ path @$/ {print $2}'
+        | xargs -I{} btrfs subvolume set-default {} /mnt
+      register: partitioning_btrfs_default_result
+      changed_when: partitioning_btrfs_default_result.rc == 0
+
     - name: Set quotas for subvolumes
       when: system_cfg.features.cis.enabled | bool
       ansible.builtin.command: btrfs qgroup limit {{ item.quota }} /mnt/{{ '@' if item.subvol == 'root' else '@' + item.subvol }}