workflows/lint: fully skip the commits job in Merge Queues

Follow-up to 7cf5972410

While the JS script already returned early, we can save a few resources
by skipping the job entirely when there's no `pull_request` context.

.github/workflows/lint.yml

@@ -126,6 +126,11 @@ jobs:
           fi
 
   commits:
+    # Only check commits if we have access to the pull_request context.
+    #
+    # Luckily there's no need to lint commit messages in the Merge Queue, because
+    # changes to the target branch can't change commit messages on the base branch.
+    if: ${{ github.event.pull_request.number }}
     runs-on: ubuntu-slim
     timeout-minutes: 5
     steps:

ci/github-script/lint-commits.js

@@ -9,8 +9,7 @@ const { classify } = require('../supportedBranches.js')
  * }} CheckCommitMessagesProps
  */
 async function checkCommitMessages({ github, context, core }) {
-  // Do not run on merge groups: it'd be much harder, and there is no need
-  // (changes to the target branch can't change commit messages on the base branch).
+  // This check should only be run when we have the pull_request context.
   const pull_number = context.payload.pull_request?.number
   if (!pull_number) {
     core.info('This is not a pull request. Skipping checks.')