Fix Focal kernel metapackage

Discovered by @emkll in [0]. The forked package build logic was
including an empty dir based on the distro string. Update the path
munging logic to use the proper var to avoid.

[0] #5772 (comment)

install_files/ansible-base/roles/build-generic-pkg/tasks/main.yml

@@ -29,7 +29,7 @@
 
 - name: Create any necessary parent directories for jinja files
   file:
-    path: "{{ build_path }}/{{ item.path | dirname |regex_replace('^\\/.*'+package_name, '') }}"
+    path: "{{ build_path }}/{{ item.path | dirname |regex_replace('^\\/.*'+package_dirname, '') }}"
     state: directory
   with_items: "{{ jinja_files_found.files }}"