Fix Archlinux variables configuration

[mike-code]

Using this role on Archlinux as-is won't work because the variables defined in Arch.yml are not included due to missing task which would include them.

This PR adds include_vars task for Archlinux distro, the same way as for other distributions.
Note that ansible_distribution_version return NA for Archlinux, thus it's not used in this fix.

console output from failed role installation

``` TASK [geerlingguy.postgresql : include_tasks] ****************************************************************************************************** included: /Users/retard/iac/ansible/_common/roles/geerlingguy.postgresql/tasks/variables.yml for vagrant

TASK [geerlingguy.postgresql : Include OS-specific variables (Debian).] ****************************************************************************
skipping: [vagrant]

TASK [geerlingguy.postgresql : Include OS-specific variables (RedHat).] ****************************************************************************
skipping: [vagrant]

TASK [geerlingguy.postgresql : Include OS-specific variables (Amazon).] ****************************************************************************
skipping: [vagrant]

TASK [geerlingguy.postgresql : Include OS-specific variables (Fedora).] ****************************************************************************
skipping: [vagrant]

TASK [geerlingguy.postgresql : Define postgresql_packages.] ****************************************************************************************
fatal: [vagrant]: FAILED! => {"msg": "The task includes an option with an undefined variable. The error was: '__postgresql_packages' is undefined. '__postgresql_packages' is undefined\n\nThe error appears to be in '/Users/retard/iac/ansible/_common/roles/geerlingguy.postgresql/tasks/variables.yml': line 22, column 3, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n\n- name: Define postgresql_packages.\n ^ here\n"}

PLAY RECAP *****************************************************************************************************************************************
vagrant : ok=2 changed=0 unreachable=0 failed=1 skipped=4 rescued=0 ignored=0

</details>

[github-actions]

This pr has been marked 'stale' due to lack of recent activity. If there is no further activity, the issue will be closed in another 30 days. Thank you for your contribution!

Please read this blog post to see the reasons why I mark issues as stale.

[fhsctv]

Maybe we should use the first_found lookup (see https://docs.ansible.com/ansible/latest/collections/ansible/builtin/first_found_lookup.html) for this to avoid having an own include task for each distribution.

[sachilles]

@geerlingguy This role cannot be used on Archlinux without this PR. Could you please accept this PR?