Backport of NET-5371 License checker pt2 into release/1.15.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #18491 to be assessed for backporting due to the inclusion of the label backport/1.15.

The below text is copied from the body of the original PR.

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Description

I created a test PR (#18486) after merging this action originally in #18485 and realized that grep behaves differently in ubuntu:latest than it does in MacOS.

This updates to use a format that works in Ubuntu and runs the action whenever new commits are pushed to a backport PR instead of just when it's initially opened.

Testing & Reproduction steps

Ran script in an Ubuntu docker container

$ docker run -it ubuntu:latest /bin/sh
$ apt update && apt install git
$ git clone https://github.com/hashicorp/consul
$ cd consul
$ ./.github/scripts/license_checker.sh
# Observe failure because many files have the BUSL header
$ git checkout release/1.15.x
$ git checkout license-checker-pt2 -- .github/scripts/license_checker.sh
$ ./.github/scripts/license_checker.sh
# Observe success because no files have the BUSL header

Links

PR Checklist

• updated test coverage
• external facing docs updated
• appropriate backport labels added
• not a security concern

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Overview of commits

• e0bdca4 - bb76a45

[github-team-consul-core-pr-approver]

Auto approved Consul Bot automated PR

[nathancoleman]

Matches source PR