headers plan: command output lists ignored files as missing headers

When running the `copywrite headers --plan` command within the Nomad repository, the output details files missing headers even though these are ignored via the [.copywrite.hcl](https://github.com/hashicorp/nomad/blob/main/.copywrite.hcl) file and the command exits with status code 0. It is a little confusing to detail these files when they are expected to be ignored.

Example output:
```
The following files are missing headers:

Using copyright holder: HashiCorp, Inc.

command/asset/*.hcl
command/agent/bindata_assetfs.go
```

Example GHA run: https://github.com/hashicorp/nomad/actions/runs/5519757257/jobs/10065442205?pr=17889