chore(packages): Change generated list of packages to default variable

Means it can be overwritten, is defined in a single place, and easier to
invoke from individual tasks (if invoking from multiple).

roles/packages/defaults/main.yaml

@@ -1,2 +1,4 @@
+all_packages_to_install: "{{ lookup('community.general.merge_variables', '^packages_.*') }}"
+
 custom_repositories:
   - https://marty-oehme.github.io/void-templates

roles/packages/tasks/main.yaml

@@ -9,5 +9,5 @@
 
 - name: Install all my used void packages
   community.general.xbps:
-    name: "{{ lookup('community.general.merge_variables', '^packages_.*') }}"
+    name: all_packages_to_install
     state: "{{ desired_package_state | default('present') }}"