Add exact time to the output of the revisions command

[sed-i]

With publishing soon being integrated with CI it would be useful to have a more detailed timestamp than just the date, because occasionally there is more than one revision per day, and it's also unclear which timezone is reported, so "the same day" might mean different things in different places.

> $ charmcraft revisions loki-k8s
Revision    Version    Created at    Status
2           2          2021-08-30    approved
1           1          2021-08-30    approved