ci(l2): do not run pr-main_l2 workflow in the merge queue (#5151)

**Motivation**

The merge queue was stuck waiting for `Integration Test L2` and `Lint
L2` jobs to pass. But the pr-main_l2 workflow is not triggered in the
merge queue, so there isn't any job running in the merge queue to meet
these GHA requirements.

**Description**

Trigger pr-main_l2 workflow in the merge queue, but skip every job to
meet the GHA requirements.

Author: ilitteri

.github/workflows/pr-main_l2.yaml

@@ -2,6 +2,7 @@ name: L2 (without proving)
 on:
   push:
     branches: ["main"]
+  merge_group:
   pull_request:
     branches: ["**"]
 
@@ -37,7 +38,9 @@ jobs:
       - name: finish
         id: finish
         run: |
-          if [[ "${GITHUB_EVENT_NAME}" != "pull_request" ]]; then
+          if [[ "${GITHUB_EVENT_NAME}" == "merge_group" ]]; then
+              echo "run_tests=false" >> "$GITHUB_OUTPUT"
+          elif [[ "${GITHUB_EVENT_NAME}" != "pull_request" ]]; then
               echo "run_tests=true" >> "$GITHUB_OUTPUT"
           else
               echo "run_tests=${{ steps.filter.outputs.run_tests }}" >> "$GITHUB_OUTPUT"