GitHub Actions - how to to get PR merge commit SHA in push?

I am using [github-actions]. I have two worksflows:

  • triggered on pull_request
  • triggered on push
on:
  workflow_dispatch:
  pull_request:
    branches:
      - master
on:
  workflow_dispatch:
  push:
    branches:
      - master

The pull_request workflow generates assets that need be used in push workflow. I need some sort of an identifier that would allow me to connect the two workflows, ideally SHA of git merge commit. However, I was not able to identify a reliable reference to use.

At the moment I am using ${{ github.event.pull_request.head.sha }} in pull_request workflow, and in push workflow I am using:

- name: Setup repository
  uses: actions/checkout@v2
  with:
    fetch-depth: 2
- name: Set CI_COMMIT_SHORT_SHA env property
  shell: bash
  run: echo "CI_COMMIT_SHORT_SHA=$(git rev-parse --short=8 HEAD^2)" >> $GITHUB_ENV

This works most of the time but breaks when commits are force pushed to the PR branch before it is merged.

What is a reliable [tag:git] reference to associate pull_request with push workflows?