Prepare use of GitHub merge queues

Merge queues should help us avoid having to merge from main repeatedly
even when all approvals are in and all CI jobs have succeeded. See https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue

.github/workflows/audit.yml

@@ -7,6 +7,7 @@
 name: Cargo Audit
 on:
  pull_request:
+ merge_group:
  push:
  # Run on changes to branches but not tags.
  branches:

.github/workflows/format-check.yml

@@ -3,6 +3,7 @@
 name: Kani Format Check
 on:
  pull_request:
+ merge_group:
  push:
  # Not just any push, as that includes tags.
  # We don't want to re-trigger this workflow when tagging an existing commit.

.github/workflows/kani.yml

@@ -3,6 +3,7 @@
 name: Kani CI
 on:
  pull_request:
+ merge_group:
  push:
  # Not just any push, as that includes tags.
  # We don't want to re-trigger this workflow when tagging an existing commit.

.github/workflows/release.yml

@@ -8,6 +8,7 @@
 name: Release Bundle
 on:
  pull_request:
+ merge_group:
  push:
  branches:
  - 'main'