Support YAML anchors and aliases

[harrydowning]

Is your feature request related to a problem? Please describe.
Once github actions supports anchors and aliases (see actions/runner#1182) this extension will need to be updated accordingly. Currently, using anchors and aliases results in errors:

Describe the solution you'd like
Using anchors and aliases should not result in these linting errors.

Additional context

name: pipeline

on: push

jobs:
  job1:
    runs-on: ubuntu-latest
    env: &env
      ENV1: env1
      ENV2: env2
    steps:
      - run: exit 0
  job2:
    runs-on: ubuntu-latest
    env: *env
    steps:
      - run: exit 0