workflows/test: test merge-group workflow

Changes to the merge-group workflow should also validate that the file
is hooked up correctly and works - otherwise we risk merging CI changes
that cause the merge queue to fail consistently.

(cherry picked from commit 534d41ee9c)

.github/workflows/merge-group.yml

@@ -2,6 +2,17 @@ name: Merge Group
 
 on:
   merge_group:
+  workflow_call:
+    inputs:
+      mergedSha:
+        required: true
+        type: string
+      targetSha:
+        required: true
+        type: string
+    secrets:
+      CACHIX_AUTH_TOKEN:
+        required: true
 
 permissions: {}
 
@@ -12,8 +23,8 @@ jobs:
     secrets:
       CACHIX_AUTH_TOKEN: ${{ secrets.CACHIX_AUTH_TOKEN }}
     with:
-      mergedSha: ${{ github.event.merge_group.head_sha }}
-      targetSha: ${{ github.event.merge_group.base_sha }}
+      mergedSha: ${{ inputs.mergedSha || github.event.merge_group.head_sha }}
+      targetSha: ${{ inputs.targetSha || github.event.merge_group.base_sha }}
 
   # This job's only purpose is to serve as a target for the "Required Status Checks" branch ruleset.
   # It "needs" all the jobs that should block the Merge Queue.

.github/workflows/test.yml

@@ -13,9 +13,11 @@ jobs:
   prepare:
     runs-on: ubuntu-24.04-arm
     outputs:
+      merge-group: ${{ steps.files.outputs.merge-group }}
       mergedSha: ${{ steps.prepare.outputs.mergedSha }}
       pr: ${{ steps.files.outputs.pr }}
       push: ${{ steps.files.outputs.push }}
+      targetSha: ${{ steps.prepare.outputs.targetSha }}
     steps:
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
         with:
@@ -45,6 +47,12 @@ jobs:
               per_page: 100,
             })).map(file => file.filename)
 
+            if (files.some(file => [
+              '.github/workflows/lint.yml',
+              '.github/workflows/merge-group.yml',
+              '.github/workflows/test.yml',
+            ].includes(file))) core.setOutput('merge-group', true)
+
             if (files.some(file => [
               '.github/actions/checkout/action.yml',
               '.github/workflows/build.yml',
@@ -63,6 +71,17 @@ jobs:
               '.github/workflows/test.yml',
             ].includes(file))) core.setOutput('push', true)
 
+  merge-group:
+    if: needs.prepare.outputs.merge-group
+    name: Merge Group
+    needs: [prepare]
+    uses: ./.github/workflows/merge-group.yml
+    secrets:
+      CACHIX_AUTH_TOKEN: ${{ secrets.CACHIX_AUTH_TOKEN }}
+    with:
+      mergedSha: ${{ needs.prepare.outputs.mergedSha }}
+      targetSha: ${{ needs.prepare.outputs.targetSha }}
+
   pr:
     if: needs.prepare.outputs.pr
     name: PR