headers plan: command output lists ignored files as missing headers

[jrasell]

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 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

[jrasell]

The default GHA setup installs a version which contains this bug, which has now been fixed in later releases.

[CalebAlbers]

@jrasell do you happen to know what version was installed and used when you encountered the bug?

I have a gut feeling this is actually due to a race condition in the part of the code that groups together log lines for GitHub Actions. The commands to start and stop logging groups were being sent to stdout, but the actual list of files were being sent to stderr, which I believe is what caused this issue. I put together #80 to fix it more generally.