ci: run pull request workflows for merge groups

[nsrip-dd]

What does this PR do?

Enables our pull request CI checks to also run on "merge groups", which are
what GitHub does to implement a merge queue.

See the official docs

Motivation

If we want to enable a merge queue, we should have our CI also run on PR
that get added to the queue. If CI fails, the PR will be removed from
the queue. Right now we require a PR to be up-to-date with main, and
have green CI, before merging into main. This basically replicates that
requirement for merge queues.

Describe how to test/QA your changes

Reviewer's Checklist

• If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• Changed code has unit tests for its functionality.
• If this interacts with the agent in a new way, a system test has been added.

[pr-commenter]

Benchmarks

Comparing candidate commit 9fce400 in PR branch nick.ripley/run-checks-on-merge-queue with baseline commit 1de4e96 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 18 metrics, 0 unstable metrics.