ensure-system-package doesn't honor system-packages
customizations (https://github.com/jwiegley/use-package/issues/661), because system-packages didn't provide an API
to retrieve shell command to be executed.
This change makes use of the new system-packages'
`system-package-get-command` function and therefore fixes the issue.
GitHub-reference: fix https://github.com/jwiegley/use-package/issues/661
Copyright-paperwork-exempt: yes