`status` command has wrong headers
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Issue body
### Bug Description
The `status` command has the header `Channel` rather than `Risk`. It also doesn't display a `Branch` column if any items are shown.
Reported initially by @tmihoc
### To Reproduce
```
$ charmcraft status my-awesome-charm
Track Channel Version Revision
latest stable - -
candidate - -
beta 0.1 1
edge ↑ ↑
```
### Environment
n/a
### charmcraft.yaml
```yaml
n/a
```
### Relevant log output
```shell
n/a
```
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