ci: add copywrite action to check file headers.

[jrasell]

This PR's copywrite CI run: https://github.com/hashicorp/nomad/actions/runs/5521733801/jobs/10070278831?pr=17889

closes #17887

[tgross]

This LGTM but will it work in the ENT repo without needing special creds? @gulducat might be able to answer that

[gulducat]

👍

this doesn't need backports though, does it?

[gulducat]

Though I don't know the full extent of what the copywrite command does, I imagine it should be fine. The contents of the repo itself are pulled by actions/checkout in the first step, which only needs this contents:read here.

Beyond that, according to the copyright repo's readme, for some operations

A GITHUB_TOKEN environment variable can be used with a Personal Access Token

I suspect the GITHUB_TOKEN (auto-provided by GHA), if used at all in our case, will mainly be to uniquely identify the caller so our private runners don't get some egress-IP rate-limited or something by GH API.

[tgross]

this doesn't need backports though, does it?

I'd have said yes but then I actually checked the release/1.5.x branch and it turns out we didn't backport all the copyright headers. So I think you're right we don't need them here.