workflows/test: run push job on correct commit

Previously, the test for the push workflow was running on the HEAD
commit of the PR only. It would be better to run it on the merged result
instead, just like any other tests we run in a PR.

(cherry picked from commit 06a0eba240)

.github/workflows/push.yml

@@ -10,6 +10,10 @@ on:
       - staging-*
       - haskell-updates
   workflow_call:
+    inputs:
+      mergedSha:
+        required: true
+        type: string
     secrets:
       CACHIX_AUTH_TOKEN:
         required: true
@@ -43,5 +47,5 @@ jobs:
     secrets:
       CACHIX_AUTH_TOKEN: ${{ secrets.CACHIX_AUTH_TOKEN }}
     with:
-      mergedSha: ${{ github.sha }}
+      mergedSha: ${{ inputs.mergedSha || github.sha }}
       systems: ${{ needs.prepare.outputs.systems }}

.github/workflows/test.yml

@@ -13,9 +13,27 @@ jobs:
   prepare:
     runs-on: ubuntu-24.04-arm
     outputs:
+      mergedSha: ${{ steps.prepare.outputs.mergedSha }}
       pr: ${{ steps.files.outputs.pr }}
       push: ${{ steps.files.outputs.push }}
     steps:
+      - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
+        with:
+          sparse-checkout-cone-mode: true # default, for clarity
+          sparse-checkout: |
+            ci/github-script
+      - id: prepare
+        uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1
+        with:
+          script: |
+            require('./ci/github-script/prepare.js')({
+              github,
+              context,
+              core,
+              // Review comments will be posted by the main PR workflow on the pull_request_target event.
+              dry: false,
+            })
+
       - name: Determine changed files
         id: files
         uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1
@@ -70,3 +88,5 @@ jobs:
       statuses: write
     secrets:
       CACHIX_AUTH_TOKEN: ${{ secrets.CACHIX_AUTH_TOKEN }}
+    with:
+      mergedSha: ${{ needs.prepare.outputs.mergedSha }}