[Feature] Provide some messaging if you are not on pull_request for GitHUb Actions CI

[orta]

Describe the bug

name: CI
on: push
jobs:
  test:
    name: Lint, typecheck & test
    runs-on: ubuntu-latest

    steps:
      - name: Danger
        run: yarn danger ci --verbose
        env:
          DEBUG: danger:*
          DANGER_GITHUB_API_TOKEN: ${{ secrets.GITHUB_TOKEN }}

Does not have danger.github.pr because it is not a PR run - however, because Danger can run on any GitHub workflow then things are going to be real surprising when it doesn't work.