Don't run SonarCloud when a change is in the merge queue

We are not interested in coverage information when a PR has already been
approved, and we were seeing flakiness with SonarCloud with the "no
artifacts found" error
element-hq/element-web#25334 .

.github/workflows/sonarqube.yml

@@ -10,6 +10,7 @@ concurrency:
 jobs:
     sonarqube:
         name: 🩻 SonarQube
+        if: github.event.workflow_run.event != 'merge_group'
         uses: matrix-org/matrix-js-sdk/.github/workflows/sonarcloud.yml@develop
         secrets:
             SONAR_TOKEN: ${{ secrets.SONAR_TOKEN }}