Document advanced usage with Merge Queue

[aldenquimby]

Github's new merge queue does not set $GITHUB_BASE_REF or $GITHUB_HEAD_REF, so the example Find modified migrations does not work

The best I've found is to use dorny/paths-filter like this:

name: Lint Migrations

on:
  pull_request:
  merge_group:

jobs:
  lint_migrations:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3
      # Works on Pull Request and Merge Queue
      - name: Find modified migrations
        uses: dorny/paths-filter@4067d885736b84de7c414f582ac45897079b0a78
        id: modified-migrations
        with:
          base: ${{ github.event.merge_group.base_ref}}
          ref: ${{ github.event.merge_group.head_ref }}
          list-files: shell
          filters: |
            migrations:
              - migrations/**/*.sql
      - uses: sbdchd/squawk-action@v1
        with:
          pattern: ${{ steps.modified-migrations.outputs.migrations_files }}