Is there a way to make GitHub CI build the test merge of a PR, rather than the head of the PR branch?

When I create a PR, GitHub CI (via the actions/checkout action) checks out the head of the PR branch. For example, if the head of the PR branch has the SHA cc87b2733dfbe579a4451b2359191a6c512207c3, I see this in the GitHub CI log:

git checkout --progress --force cc87b2733dfbe579a4451b2359191a6c512207c3

Whereas other CI systems check out the test merge of the PR. For example, if the PR number is 123, in the Travis CI log I see:

git fetch origin +refs/pull/123/merge
git checkout -qf FETCH_HEAD

And in the Appveyor log I see:

git fetch -q origin +refs/pull/123/merge
git checkout -qf FETCH_HEAD

Is there a way to make GitHub CI build the test merge of a PR, rather than the head of the PR branch?

As per https://github.com/actions/checkout/issues/15:

The GitHub docs are misleading:

If you intend to use the pull_request event to trigger CI tests, we recommend that you set up your workflow configuration to listen to the push event.

I've discovered that the following works exactly as I want:

on:
  push:
  - master
  - release-*
  pull_request:

I was concerned that I would get too many builds triggered using the pull_request event, since according to the docs:

Triggered when a pull request is assigned, unassigned, labeled, unlabeled, opened, edited, closed, reopened, synchronize, ready_for_review, locked, unlocked or when a pull request review is requested or removed.

But it seems that GH actions is clever enough not to trigger a rebuild of a commit that was already built.