We rely on charm-env so the cron job can find dependent libraries/modules. Unfortunately, with a unit with multiple charms (subordinates), charm-env fails with 'Ambiguous possibilities for JUJU_CHARM_DIR'. This fixes that by specifying we want the 'ubuntu-repository-cache' charm and the bundled libraries/modules.
Reviewed-on: https://code.launchpad.net/~hloeung/ubuntu-repository-cache/metadata-sync-cron-charm-env-fixes/+merge/387088 Reviewed-by: Paul Collins <paul.collins@canonical.com>