bundles/apt: allow variables {os} and {os_release} in sources.list entries

bundles/nginx/metadata.py

@@ -3,13 +3,13 @@ defaults = {
         'repos': {
             'nginx': {
                 'items': [
-                    'deb http://nginx.org/packages/debian buster nginx',
+                    'deb http://nginx.org/packages/{os} {os_release} nginx',
                 ],
             },
         },
         'unattended-upgrades': {
             'origins': {
-                'o=nginx,a=stable,n=buster,l=nginx,c=nginx',
+                'o=nginx,a=stable,l=nginx,c=nginx',
             },
         },
         'packages': {